SUMMARY, CONCLUSION AND FUTURE SCOPE OF THE WORK

91 CHAPTER VII SUMMARY, CONCLUSION AND FUTURE SCOPE OF THE WORK 7.1 Summary Chapter 1 describes the importance and motivation behind the evaluation ...
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CHAPTER VII

SUMMARY, CONCLUSION AND FUTURE SCOPE OF THE WORK 7.1 Summary Chapter 1 describes the importance and motivation behind the evaluation of thermal characteristics for the MMC’s as well as TIMs. Various methodologies of manufacture of the MMC’s are discussed along with the advantages and limitations. The reinforcements in MMC’s are explained in details and the corresponding materials are listed. Thermal contact resistance in heat transfer applications are presented with examples. The heat transfer phenomenon at the interfaces is detailed with the classification based on contact criteria. In Chapter 2 literature available in the area of metal matrix composites in thermal management is presented. The literature review is grouped as Thermal properties of MMCs and Processing of metal matrix composite materials for the sake of better organization of the review. The literature of the thermal interface materials are review and are studied for thermal grease, thermal pads and other thermal interface materials. Finally literature review of TCR models are given for single contact spot, surface roughness and deformation of contact spots. Important conclusions are drawn from the literature review. Chapter 3 deals with objectives and methodology of the research work. Initially, scope of the research work is described followed by objective of the research work and methodology of the research work. Sources of data and techniques of the research work are also given at the end of the chapter.

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Chapter 4 is dedicated for the development of new MMC’s. For this initially, baseline materials are explained in detail along with their thermal properties. Proposed materials and compositions are given in this chapter. The experimental results for the evaluation of the thermal, mechanical and physical properties are tabulated and plotted. Chapter 5 deals with the evaluation of measurement system for contact resistances. Application of thermal greases is given in detail. The thermal management of the contact resistance models are presented with equations. The measurement system is established by conducting the experiments. In Chapter 6 development of new contact resistance models is presented with the design and experimental evaluation for the properties. The measurement system validated in chapter 5 is used for measuring the thermal contact resistance. In Chapter 7, the summary, conclusions are drawn and future work is defined.

7.2 Conclusions After review of the literatures available, it has been observed that there is not enough research work has been done to enhance the heat transfer by studying the effects of improving both the materials of heat sinks as well as the contact resistance models. In this study work, to improve the designs of the TCR models, the thermal characteristics are evaluated for a wide range of compositions of the MMC’s as well as thermal interfacial materials. The effect of the composition on the thermal properties is studied in order to come out with a means to enhance the transport characteristics of heat sinks.

The experimental evaluation of the thermal characteristics of different composition of new MMC’s developed in this work in Table 4.9. It shows the summary of the properties of the new MMCs developed are presented. The study indicates that the MMC

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35Al-65SiC has a very good thermal conductivity and 25Al-75SiC has the least CTE, 65Al-35SiC has the highest Specific heat among all the MMC’s. One has to choose the composition based on the requirement. For example, for the heat sink applications, one can choose 30Al-70SiC because it has good thermal conductivity and CTE both. The variation of thermal conductivity for different compositions. From the results presented , it is concluded that 50 % of SiC acts as a deterrent for the thermal conductivity any amount of increased in the aluminum does not contribute to the enhancement of thermal conductivity. This is due to the reason that the distribution of the SiC particles occupies at least 50% of volume so that it acts as a barrier for the heat flow. The variation of thermal conductivity for different compositions. The specific heat capacity increases with increased in the volume percentage of the aluminum from 25 to 65. However, the relationship is not linear. It is observed that by increasing the percentage of aluminum content the specific heat of MMC increases.Figure 4.3 shows the variation of CTE for different compositions. With increase in the volume percentage of the aluminum from 25% to 65%, the specific heat capacity increases. It is clear that by increasing the percentage of aluminum content, the CTE of the MMC increases. The variation of Young’s modulus for different compositions. With increase in the volume percentage of the aluminum from 25 to 65, the young’s modulus decreased gradually and becomes stagnant. The relationship is not linear. The variation in modulus of elasticity is observed only when the SiC is varied between 55% and 70%. Beyond these values, the modulus of elasticity becomes stagnant due to the fact that, one material forms the matrix and the other forms the constituent leaving the net effect unchanged. The variation of density for different compositions. With increased in volume percentage of the aluminum from 25% to 65%, the density decreases. Density along with

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specific heat need to be considered together while optimizing the material selection. The density of aluminum is 2.7 gm/cc and that of SiC is 3.2 gm/cc. Hence with increase of SiC, the net density of the MMC increases. The variation of thermal conductivity with respect to temperature. When operating at temperatures up to 150°C, one can choose 30Al-70SiC if the thermal conductivity is criteria for selection, since it performs well at temperatures except near room temperature under study. The variation of CTE with respect to temperature. Due to practical limitations of the measurement facilities, the properties are not measured beyond 150°C. When operating at temperatures up to 150°C, one can choose 25Al-75SiC if the CTE is a criteria for selection, since it performs well at temperatures under study. The variation of specific heat of MMCs with respect to temperature. Due to practical limitations of the measurement facilities, the properties are not measured beyond 150°C. When operating at temperatures up to 150°C, one can choose 65Al-35SiC if the specific heat is a criteria for selection, since it performs well at all temperatures under study. The variation of thermal diffusivity of MMCs with respect to temperature. When operating at temperatures up to 150°C, one can choose 30Al-70SiC if the Thermal diffusivity is a criteria for selection, since it performs well at temperatures under study. Overall, the thermal properties are evaluated for MMCs and based on the section criteria, the composition of the MMCs can be chosen for the best performance. The measurement systems for measuring the thermal contact resistance wasvalidated and the thermal contact resistance of the thermal greases was validated with

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respect to the theoretical values. Experimentations were conducted for each set of three materials. The thermal contact resistance compared three sets of materials and found least one for each set, the measured values of thermal contact resistance compares very well with the theoretical prediction. New contact resistance models were developed and the measurement systems which are validated in Chapter 5 were used to measure the contact resistances. Three different types of models are verified in this work, namely, PGH, PG1G2H and PG1FG2H. In the PGH model, grease G is high conducting grease or low conducting grease. It is proved experimentally that by adding additional highly conducting layers the thermal resistance drops. Also it is verified that by reducing the thickness of the layer next to the processor, the thermal conductance improves. The purpose of enhancement in the characteristics of the thermal management systems are achieved by two means, namely,  Development of

new MMC’s for heat sinks to match closely with the

required properties of the heat source  Development of new thermal contact resistance models to overcome the short comings of the thermal management systems being used without thermal grease/pastes. The thermal management consists of three components, namely, heat source (CPU), heat sink, and interstitial materials to fill the gap between heat source and heat sink. To enhance the performance of the thermal management system, design of CPU cannot be altered by the user as its design and performance is decided by the manufacturer of the CPU. The user can only modify the designs of heat sink and the interstitial elements to enhance the overall performance. In this research work, effort is made towards this goal.

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The composition of the heat sink materials are altered in such a way that the required properties can be achieved. Overall, six compositions are produced with varying percentages of Al and SiC. They are: 

25Al-75SiC



30Al-70SiC



35Al-65SiC



45Al-55SiC



55Al-45SiC



65Al-35SiC

The thermal and mechanical properties of these materials are measured and the material to be chosen based on the required property criteria of the heat source material. The following conclusions are drawn: Out of the experiments conducted, it is concluded that the MMC 35Al-65SiC has a very good thermal conductivity and 25Al-75SiC has the least CTE, 65Al-35SiC has the most Specific heat among all the MMCs. One has to choose the composition based on the requirement. For example, for the heat sink applications, one can choose 30Al-70SiC because it has got a good thermal conductivity and CTE both. Table: 7.1 Composition chosen for specific applications Criteria

MMC to be chosen

Thermal Conductivity

30Al-70SiC

CTE

25Al-75SiC

Specific Heat

65Al-35SiC

Thermal diffusivity

30Al-70SiC

Density

65Al-35SiC

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Young Modulus

25Al-75SiC

The other way of improving the performance of the thermal management system is by developing new thermal resistance models. Three thermal resistance models are developed namely, PGH, Pg1G2H and PG1FG2H. P stands for Processor, G stands for grease and F and H stands for foil and heat sink respectively. It is concluded that the models PGH with Al-Foil, PGH with G2 and PG1FG2H has the lower temperatures at the heat sink interface for a heat removal of 110W. G1 grease has lower thermal conductivity and G2 has higher thermal conductivity than G1. But as mentioned before the interfacial material which is adjacent and in touch with the CPU must have the lowest thermal conductivity to avoid the shorting on electronics on the CPU. Hence the model PG1FG2H is treated as the best case for this application.

7.3 Future Scope of The Work The following research work may be continued in the future for further development:  Evaluation of thermal properties by varying the size of the Aluminum particles.  Evaluation of thermal properties by varying the size of the SiC particles.  Development of technology to improve adhesive properties of the heat sinks while increasing the thermal transportation characteristics.  Measurement of the heat transfer for the heat sink with the material and contact resistance models developed in this research work.  Development of MMCs with any other base materials and constituents which has better properties than Al and SiC MMCs.