FIELD OF THE INVENTION

The present invention relates generally to a novel composite material, which is particularly suitable for devices used to cool components such as heat sinks, for materials to regulate temperature and for applications where the volume change that accompanies the phase change is used for actuator applications and more particularly the present involves a technique of electrodeposition or electroless deposition to incorporate phase change material (PCM) in a metal layer, for instance a copper, nickel or zinc layer

BACKGROUND OF THE INVENTION

Integrated circuits such as computer CPU's often produce a lot of heat, which should be dissipated as quickly as possible since failure of integrated circuits increases with their working temperature. Since integrated circuits often work in a boolean manner, switching on and off, thermal loads are often transient. This gives rise to thermal fatigue, which also reduces the lifetime of integrated circuits. Therefore, the design of heat sinks is very important. The ideal material for heat sink applications combines a high thermal conductivity with a high heat capacity to adsorb thermal spikes. Metals have a high thermal conductivity and hence are commonly used as heat sinks, but their heat capacity is low.

Therefore, recently new heat sinks were designed which combine metals with phase change materials (PCM's) [P. Gauche & W. Xu, Int. Conf. on high-density interconnect and systems packaging, Denver CO, 402-406 (2000)]. PCM thermal energy storage is based on the latent heat adsorbed or released when a material reversibly changes phase, usually between the solid and liquid state. During this phase change, the temperature of the PCM stays constant, which is ideal to adsorb thermal spikes. Typical PCM's have melting temperatures between -15° C and 190° C [B. Zalba, et al Appl. Thermal. Eng., 23, 251 -253 (2003)] which comprises the range of temperatures useful for cooling of integrated circuits. The latent heat of most PCM's tends to range between 150 and 250 kJ/kg [B. Zalba, et al Appl. Thermal. Eng., 23, 251-253 (2003)].

Although the new heat sinks with PCM's have superior thermal properties, their performance is not optimal due to the fact that they combine metal with bulk PCM's. Since the thermal conductivity of PCM's are low, there is a need in the art for better thermal properties. Novel metal matrix composite material containing PCM particles are an embodiment of the present invention. Better thermal properties of said novel metal matrixes are obtainable by finely and homogeneously embedding said PCM's in the metal by means of an electrodeposition or electroless process. The PCM's are deposited by electrodeposition or by electroless deposition in the metal. ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 displays a SEM picture of micro-encapsulated paraffin particles with a urethane shell. The particles were cryogenically fractured to show the thickness of the urethane shell.

Figure 2 shows the DSC measurement of pure paraffin and micro-encapsulated paraffin. The measurement was done at 10 °C/min.

Figure 3 demonstrates a SEM picture of a cryogenic fractured copper coating with microencapsulated paraffin particles. This figure shows that the incorporated amount of paraffin particles is very high.

Figure 4 displays a SEM picture of a cross-sectioned copper coating with embedded microcapsuled paraffin particles.

Figure 5 demonstrates the curve of DSC measured on a copper coating with embedded paraffin microcapsules. The measurement was done at 10°C/min.

Figure 6: Strain of composites ECDO to ECD3 composed of paraffin containing microcapsules embedded in a copper coating as a function of temperature as measured by a dilatometer. The amount of strain is a function of the amount of PCM material in the coating.

SUMMARY OF THE INVENTION

In accordance with the purpose of the invention, as embodied and broadly described herein, the invention is broadly drawn to a composite material comprising a metal matrix containing phase change material particles or microcapsules containing phase change material, wherein said composite materials is obtainable by electrocodeposition or electroless deposition of metal with the phase change material particles or microcapsules containing phase change material. The composite material can be obtainable by electrodeposition or electroless deposition of said phase change material particles or microcapsules with a metal and the amount of said phase change material particles or microcapsules added to the electrolyte is an amount between 1 to 500 g/l and the electrocodeposition has been carried out at current densities ranging from about 0.1 to about 50 A/dm ² or the electroless deposition has been carried out at temperatures ranging from 10 to 95 ⁰ C By this method the phase change material particles or microcapsules containing phase change material are homogeneously distributed in the composite material.

In one aspect of the invention, the composite material comprises a metal matrix comprising phase change material, designed to undergo a phase change and hence a volume change. These microcapsules can comprise a paraffin core and a urethane shell.

In another aspect of the invention is the composite material have a metal matrix which comprises more than 10 vol%, more than 20 vol%, between 20 and 35 vol%, about 35 vol%, more than 35 vol% or between 35 or 40 vol% of said phase change material particles or microcapsules containing phase change material.

In still another aspect of the invention the composite material comprises phase change material particles have mean sizes ranging from 1 to 100 μm and optionally have a shell thickness of about 0.5 to 5 μm. The phase change material particles can comprise paraffin that is a mixture of short and long chained alkanes with melting points of respectively about 41 ⁰ C and about 59° C.

In yet another aspect of the invention the composite material of present invention is used to coat a layer on a substrate. This coating may have a thickness of up to 50 μm, of more than 50 μm, of 10 μm to 500 μm or of 50 μm to 300 μm. Such coating may comprise a phase change material having a latent heat of more than 10 J/g or may comprising phase change material having a latent heat of about 10 J/g. Such coating

combines the high thermal conductivity of metals with the high heat absorption capacity of phase change material particles.

Such composite material and coating is particularly suitable for manufacturing a heat sink means or an actuator means

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof. It will be apparent to those skilled in the art that various modifications and variations can be made in in construction of the system and method and more particularly in composite coatings using the technology of incorporating phase change material (PCM) particles or microcapsules containing phase change material in a metal of the present invention or in the use of the composite coatings or of devices comprising the composite coatings without departing from the scope or spirit of the invention.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only.

A unique way to prepare such coatings is codeposition of the metal matrix and the PCM particles, more particularly by electrocodeposition, a process where particles are incorporated in metal deposited by electrolysis or by electroless deposition.

Micro-encapsulation can be done for various reasons, depending on the choice of PCM. For sticky PCM's such as paraffin waxes, micron sized particles agglomerate during their preparation if they are not encapsulated. For other PCM's such as hydrated salts, micro-encapsulation might be used to prevent contact between the PCM and the metal in composite coatings, since this would lead to corrosion of the metal coating.

For present invention copper was used as the matrix material since copper has a heat conductivity of 400 VWmK, which is high, compared to most other metals [R. Weast & M. Astle, CRC handbook of chemistry and physics, 63rd edition, CRC Press, Boca Raton (1982)]. As a phase change material, paraffin with a melting point of 54° C as specified by the manufacturer was chosen because of its high latent heat, good thermal stability and non-toxicity [B. Zalba, et al Appl. Thermal. Eng., 23, 251 -253 (2003)]. Paraffin microcapsules with a urethane shell were synthesized by emulsification of a solution containing molten paraffin, toluene-2,4- diisocyanate, 4,4'-foryldiphenol and dibutyltin dilaurate in an aqueous solution of polyvinyl alcohol.

When the emulsion is heated, the urethane shell around the molten paraffin droplets forms by interfacial polycondensation of toluene-2,4-diisocyanate and 4,4'- foryldiphenol. Dibutyltin dilaurate acts as a catalyst for this reaction. Using this procedure, particles with mean sizes ranging from 6 to 15 μm were obtained. Figure 1 shows a scanning electron microscopy (SEM) picture of paraffin microcapsules. The particles were cryogenically broken to show the thickness of the urethane shell. Typically, particles with a shell thickness of about 1 to 2 /vm were obtained. Ideally, the urethane shell around the particles should be as thin as possible because the thermal conductivity of the shell is low (typical thermal conductivity for polymers 0.2 to 0.4 W/mK [J. Bantrup, et al Polymer handbook, 4th ed., John Wiley & Sons Inc., New York, NY (1999)]). Therefore, heat conduction through the shell goes more slowly for thicker shells and hence it takes more time before the PCM becomes effective when the shell thickness is increased. Actually, to obtain the most performant thermal properties of metal composite coatings with incorporated PCM particles, the particles should have no polymeric shell at all. However, as already mentioned, this is not always feasible due to the difficulties of producing pure PCM particles or due to chemical incompatibilities between the PCM particles and the metal coating. The thermal properties of paraffin microcapsules prepared in this work were compared with the thermal properties of pure paraffin by differential scanning calorimetry (DSC) measurements. The results are shown in Figure 2. It was found that the DSC curve for pure paraffin has two peaks,

which indicates that the paraffin is a mixture of short and long chained alkanes with melting points of respectively 41 ⁰ C and 59° C. The latent heat determined from the integration of the peaks in the DSC curve is 291 .4 J/g. This corresponds well with values of the latent heat for paraffin found in literature [B. Zalba, et al Appl. Thermal. Eng., 23, 251 -253 (2003)]. The dashed base line for the DSC measurement of paraffin in Figure 2 is almost horizontal which indicates that the heat capacity of paraffin is low. This is in agreement with literature where a value of 2.9 J/gK is found [S. Goodhew & R. Gri.ths, Appl. Energy, 77, 205-223 (2004)]. The total heat Q which the paraffin takes up is given by

Q = Lm + f ² mcp dT ^• (1)

with L the latent heat, m the mass, cp the heat capacity and T the temperature. The term with the heat capacity varies linear with temperature if cp is temperature independent and corresponds with the dashed base line in Figure 2. When cp is small, the contribution of this term to the total heat flow is small and the dashed line is almost horizontal. Compared with pure paraffin, the latent heat for micro-encapsulated paraffin is smaller. The reason for this is the fact that only the paraffin undergoes phase transformation while the urethane shell stays solid. Hence the urethane shell adsorbs almost no heat because its heat capacity is low [J. Bantrup, et al 4th ed., John Wiley & Sons Inc., New York, NY (1999)]. The heat capacity of the urethane shell ( 1.3 J/gK [B. Vankatesan, et al. Int. J. Therm. Sci., 40, 133-144 (2001 )]) is comparable to the heat capacity of paraffin, which is reflected in the horizontal base line of the DSC curve (dashed line in Figure 2). If the heat capacity of the urethane shell will be much larger than the one of paraffin, the slope of the dashed base line will be larger for micro¬ encapsulated paraffin than for pure paraffin. The DSC results of the micro-encapsulated paraffin particles also show two peaks, corresponding with two melting temperatures. However, the peaks occur at slightly higher temperatures than for pure paraffin. This is due to the fact that the urethane shell around the paraffin has a large thermal resistance. Therefore, a larger temperature gradient is necessary to transmit heat from the outside to the inside of the paraffin microcapsules, which implies that melting occurs at a higher temperature. Moreover, when pure paraffin particles start to melt, they flow onto each other, which improves thermal conduction. Micro-encapsulated paraffin particles keep their shape during melting and hence the thermal conductance

stays low. This also contributes to the higher melting temperature of micro¬ encapsulated paraffin with respect to pure paraffin.

To make copper composite coatings with homogeneously distributed paraffin particles, paraffin microcapsules were dispersed in a copper sulfate plating electrolyte from which copper coatings were electrodeposited at room temperature.

Electrodeposition was done at current densities ranging from 1 to 5 A/dm ² and coatings with thickness up to 50 μm were made. However, thicker coatings can also be prepared. Depending on the deposition parameters, coatings with 35 vol% of incorporated phase change material can be easily obtained. Figure 3 shows a SEM picture of a cryogenic fractured coating with such high particle incorporation. Almost the whole fracture area consists of cavities where particles had been positioned which fell out of the coating during fracturing. However, the picture clearly shows that the particles are homogeneously distributed and that the incorporation of paraffin particles is very high. The volume fraction of paraffin particles in the coating is 39 vol% as determined by the surface fraction of particles on a polished cross section (Figure 4). To investigate the thermal properties of coatings with paraffin microcapsules, DSC measurements were done. A result of such measurement is given in Figure 5, which is a DSC curve of a copper coating containing 35 vol% of paraffin microcapsules. The DSC curve shows peaks at 40.1 ° C and 82.7° C. The first peak occurs around the same temperature as for free paraffin microcapsules, while the second peak occurs at a much higher temperature than the one found for free microcapsules. The difference in melting temperature between embedded and free microcapsules might be due to the difference in thermal expansion of the paraffin microcapsules and the copper coating. When the coating is heated, it exerts a pressure on the paraffin particles which is described by the following equation [T. Mura et al. Martinus Nijholt. publishers, Dordrecht (1987), T. Clyne & P. Withers, Cambridge University Press, Cambridge (1993).]

with Δα the difference in thermal expansion of copper and paraffin, ΔTthe temperature range over which the composite coating is heated, vm and vp the Poisson ratio's of copper and paraffin and Em and Ep the elastic moduli of copper and paraffin. For typical

values, the equation predicts a pressure of 52 atm for a coating heated from room temperature to 64° C which is the melting temperature of the paraffin microcapsules. This pressure leads to an increased melting temperature according to the Clausius- Clapeyron equation [B. Kyle, , 2nd ed., Prentice Hall Inc., Englewood Cli.s, NJ (1984).]

dP (3)

with T the equilibrium temperature during melting, AV the volume change during melting and AH the change in enthalpy with phase transformation. For paraffin microcapsules with a melting temperature of 64° C, the above equation predicts an increase of the melting temperature with pressure of about 0.383 K/atm. Hence, the pressure of 52 atm, which the coating exerts on the particles, increases the melting temperature from 64° C to 84° C which is in correspondence with the DSC measurement. When designing new composite coatings with a certain melting temperature, the pressure dependency of the melting temperature should hence be taken into account. The latent heat of the copper coatings with 35 vol% of incorporated paraffin microcapsules was 10.9 J/g. However, potentially, the latent heat of composite coatings can be increased to about 20 J/g if paraffin microcapsules with thinner shells will be used.

A back of the envelope calculation shows that coatings with a latent heat this high are useful for heat sink applications. For instance, a Celeron" processor (Intel corporation) produces 21.7 Watt of heat. When this processor works continuously during 10 seconds, 217 Joule of heat are produced which have to be dissipated. Two coatings with the same mass are compared: a pure copper coating and a copper coating containing 35 vol% of paraffin microcapsules. The experimentally determined value of the latent heat of the paraffin microcapsules is 178 J/g, the heat capacity of the paraffin microcapsules is 2.9 J/gK according to literature [S. Goodhew & R. Gri.ths, Appl. Energy, 77, 205-223 (2004)] and the heat capacity of copper is 0.385 J/gK [R. Weast & M. Astle, CRC handbook of chemistry and physics, 63rd edition, CRC Press, Boca Raton (1982)]. A coating with a mass of 8.9 g was used for the calculations, which corresponds with a pure copper coating having a surface area of 10 by 10 cm and a thickness of 100 μm. For a composite coating with a total mass of 8.9 g and 35 vol% of paraffin microcapsules, the mass of copper is 8.4 g and the mass of paraffin is

0.5 g assuming that the density of the particles is 1 g/cm ³ . The temperature increase for the pure copper coating and the composite coating is calculated according to the following equation

Q (4)

with m' _Cu the mass of the pure copper coating, m _Cu the mass of copper in the composite coating, mp the mass of the paraffin microcapsules in the composite coating, V the heat capacity of copper, ^c £ the heat capacity of the paraffin microcapsules and Lp the latent heat of the paraffin microcapsules. For Q = 217 J, a Δ7'of 66° C is found for the pure copper coating while only a Δ7 of 29° C is found for the composite coating provided that the end temperature is above the melting temperature so that phase transformation took place. Although the above values give an indication of the temperature reduction by using phase change materials, the calculation was only indicative. In real cases, the phase change material, which is melting, adsorbs the heat and since the melting front moves, the conduction of heat in the phase change material also plays a role. Also the fact that phase change material is distributed in the metal complicates exact calculations of the heat flow.

By present invention it was demonstrates that metal matrix composite coatings with PCM microcapsules can be designed which are attractive materials for heat sink applications since they combine the high thermal conductivity of metals with the high heat absorption capacity of PCM's. The high heat absorption capacity occurs in the melting range of the PCM. Therefore, incorporation of a mixture of PCM microcapsules with different melting ranges should be done in future work to expand the temperature range at which heat is adsorbed in order to obtain coatings with even better thermal properties.

The composite PCM material of this invention also has interesting actuator properties. Figure 6 shows the thermal properties of the composites EDCO to EDC3 as determined with a dilatometer by cycling the temperature of cylindrical samples between -10 and 150 ⁰ C. Table 1 shows the deposition conditions and the volume fraction of codeposited microcapsules as determined by carbon analysis. As can be seen, the samples with microcapsules show a sudden increase in the strain at the melting point of

the paraffin and the amount of strain of the samples increases with increasing concentration of microcapsules in the coating.

METHODS

Preparation of PCM microcapsules

The formation of PCM particles with an urethane shell occurs by polycondensation of toluene- 2,4-diisocyanate (TDI, Merck) and 4,4'-foryldiphenol (DDS, 99.9 %, Acros) in presence of the catalyst dibutyltin dilaurate (DTD, 95 %, Aldrich). Practically, the synthesis was done by emulsification of an oily phase in an aqueous phase. The aqueous phase was made by dissolving 1 g polyvinylalcohol (PVA, 80 % hydrolyzed, MW 8000-10000, Aldrich) in 400 ml water at 60° C. The oil phase was made by dissolving 40 g of DDS in a solvent mixture composed of 40 g tetrahydrofurane (THF, Acros) and 2 g aceton. The resultant solution was heated to 60° C and thereafter, 50 ml of molten paraffin (melting point 54° C, VeI) was added. Finally 12 g of TDI and 12 drops of DTD were added. During emulsification of the oily phase using an ultraturrax (Janke & Kunkel, Ika-labortechnik), paraffin/THF droplets in an aqueous PVA solution are formed. The emulsion was kept at 65° C during 1 hour while gently stirred. During heating, the TDI and DDS reacts to form urethane. The urethane is expelled to the interface between the aqueous phase and the paraffin/THF droplets while the low boiling THF solvent evaporates, and a urethane shell is formed. After completion of the reaction, a dispersion of encapsulated paraffin particles in an aqueous PVA solution remains. This solution was washed 6 times with demineralized water of 60° C.

Codeposition of PCM

Copper deposits with a thickness of 50 //m were made at room temperature from a copper electrolyte composed of 200 g/l CuSCk.5H2O and 50 g/l H2SO4. This was done at current densities between 1 and 5 A/dm ² . The amount of particles added to the electrolyte was varied between 50 and 350 g/l. Surfactant was added to the electrolyte in order to make the particles less hydrophobic and to prevent .coagulation. The cationic surfactant, cetyltrimethylammonium hydrogensulfate (CTAHS, 99%, Aldrich) was used for this purpose. Deposits were made on stainless steel substrates (Folien- Band, thickness 0.250 mm, Overtoom International Belgium N.V., Belgium), which were

vertically suspended in the electrolyte. Prior to metal deposition, the substrates were degreased with a commercial alkaline cleaning solution (VR 6334-16, Henkel), rinsed with demineralized water, dipped in a solution of 10 % HCI and rinsed again with demineralized water. The substrates had a surface area of 1 cm ² . During codeposition, the electrolyte was stirred with a paddle stirrer to keep the particles well suspended.

Differential scanning calorimetry

Samples for DSC measurements were weighed with a precision of ± 0.01 mg on a microbalance (Mettler AE2400) and placed in aluminum pans with coverlids. The temperature in the measurement chamber was equilibrated at -20° C, using liquid nitrogen as a coolant and thereafter, DSC measurements were done at a constant heating rate of 10° C/min up to a temperature of 120 ⁰ C.

Dilatometer measurements

The thermal expansion of cylindrical samples was measured with a vertical dilatometer Q400 of TA Instruments. During the measurement, the sample is heated at 2 °C/min between -10 and 150 ⁰ C. The force on the sample was 0.05 N.

Table 1 : Parameters for the electrodeposition of paraffin microcapsules from a copper sulfate bath and results of the carbon analysis for the samples ECD1 to ECD3.