| The MicroElectric and PhotoElectric industry develops toward high level of integration,high-power and high-performance and it results in a high density heat flux problem whichreduces the performance of chips. The traditional heat exchangers are difficult to accord withthe heat flux environment at present and the effective heat dissipation of the chip is importantto ensure their stable work. Micro Heat Pipe(MHP), which working principle bases on phasechange, is a major solution to this problem. MHP technology is increasingly becoming themain way of cooling of microelectronic components and systems. Therefore, it is the key tostrengthen the MHP heat transfer efficiency in the Micro-electric field. The functional surfacewith the axial ladder contact angle distribution can increase of the liquid capillary forces aswell as not obvious increase the liquid shearing force, which brings about the increase incondensate mass flow rate through the adiabatic section of MHP. The wettability gradientsurface has excellent properties, but it is hard to be prepared in MHP. Therefor, itsconstruction technology based on the copper substrates needs to be studied. In this paper, thewettability gradient surfaces on the copper substrate are prepared by two kinds of process anddifferent functional surfaces are constructed on the groove structure in MHP. The increasedthermal performance associated with the functional surface is more obvious. The mainlyresearch works are summarized as follows:First of all, a series of random (meth)acrylate polymers were prepared by the free radicalsolution polymerization and their coating film wettability was analyzed by measuring watercontact angles(CAs). The relationships between (meth)acrylate polymers structure and its filmsurface wettability were analyzed. There are three kinds of functional groups on the surface,including methyl, ethyl and ester group, and the surface energy of ester group is greater thanthe others. The more ester group on the surface will result in the better surface bydrophilicity.Polymer structure parameters as the side chain length(m) and glass temperature(Tg) are takeninto account in discussing the migration of functional group to the outermost surface. It wasfound that the less m and lower Tg will promote the motion of the side chain and hence causethe surface exhibiting more hydrophilicity. Secondly, a gradient wettability film on copper slide was obtained by a simple controlledester group hydrolysis procedure of poly(methyl methacrylate-butyl acrylate)[P(MMA-BA)]films coated on the copper surface. A series of P(MMA-BA) copolymers with different MMAcontents were synthesized by a conventional free atom radical solution polymerizationmethod. In this section, surface wettability gradients were prepared in poly(MMA-BA) by asodium hydroxide solution. Surface chemical composition from the ester group to acid saltwas achieved owing to the ester group hydrolysis of the alkaline solution concentration andreaction time. The physicochemical properties of the resulting surfaces were extensivelycharacterized. The field-emission scanning electron microscopy (FSEM), atomic forcemicroscopy (AFM) and X-ray photoelectron spectroscopy (XPS) results showed that thevarying concentration along the substrate length was only attributed to the hydrolysis reactionof ester groups and the physical properties remaining insignificantly altered. It is found thatthe BA copolymer content can influence the content of ester groups on the outermost surfaceand hence the slope gradients.In addition, in order to improve the alkali resistance of P(MMA-BA) films, HEMA waschosen to as a crosslinking monomer. The thermosetting copolymer P(MMA-BA-HEMA)was prepared and the amino resin CYMEL303was a curing agent. By increasing the alkaliconcentration and reaction time, the copolymer film surface can exhibit the wettabilitygradient and the contact angle slowly decreases from89.3°to19.9°, which increased therange of contact angles(CAs).Thirdly, the preparation of gradient wettability surfaces on copper substrates under mildcondition was discussed. The functional surfaces were directly fabricated on copper slides bythe oxidation technique in aqueous solutions of sodium hydroxide and ammonium persulphateat ambient temperature and pressure. The contact angles on copper surface can be controlledby varying the immersion time. The wettability of the surface was changed fromhydrophobicity to superhydrohilicity gradually. The morphologies, chemical composition andstates, and wettings of the film on the copper slide substrate were analyzed by means offield-emission scanning electron microscopy (FSEM), X-ray photoelectron spectroscopy(XPS) and water contact angles (CAs) measurement. It was found that a series of nanostructure arrays had been grown on the copper surfaces and the rough morphologydecreased the contact angles. Wenzel equation can be cited to illustrate thesuperhydrophilicity of a roughened surface, which is usually used to theoretically explaincapillary phenomenon. As we know, copper as a kind of important engineering materials hasbeen widely used in many industrial applications. The novel method is expected to besignificant for the preparation of gradient wettability materials with high efficient heattransfer and the controlled procedure is convenient, cheap and environmentally friendly.Fourthly, a model is presented for predicting liquid flow velocity in a rectangularmicrochannel driven by capillary force, gravity and an extra driving force due to the surfacewettability gradient. The nth power function,cos cos0(cos cos0)(xx LL)n, for thecosine of contact angle (CA) with typical n order of0.5,1and2is applied to analyze how theinner surface wettability gradient of the microchannel affects the flow velocity. Flowsimulations revealed that the velocity of liquid flow decreases with the length ofmicrochannel and the wettability gradient (e.g. from80°to2°) on channel surface willaccelerate the motion of liquid when the flow-front approaches to the end of the microchannelalthough the gradient surface may decrease the initial motion of liquid due to the great CA atchannel entrance in comparion to the uniform CA channel（with respect to the lowest CA of2°）. The linear function (n=1) of wetting gradient profile may achieve relatively more stableand higher flow velocity than the other n power functions. The analysis of driving force alongthe moving path matches well with the flow velocity predicted by the model. If the extradriving force due to the suface wettability gradient dose not exist, the functional surface canalso decrease the liquid shearing force in gas-liquid reverse flow, which brings about theincrease in liquid mass flow rate through the microchannel.Finally, the different wettability gradient surfaces were constructed on the MHP innersurface by the solid-liquid interface oxidation and the thermal performance of MHP with thefunctional surface was measured. The experimental results show that the wettability gradientsurface can effectively reduce the thermal resistance and increase the thermal performance ofMHP. The functional surface can strengthen the heat transfer effect of MHP with groovestructure. |
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