The use of a computer is an inevitable part of our daily lives. Overheating computers is a problem that affects productivity on a daily basis. When a computer is heavily used for long periods of time or heavy software and games are run, it lags and sometimes throttles due to overheating. A 1°C increase in the temperature of a device above its operating temperature results in a 4% reduction in reliability [1]. The high temperature of portable devices (laptops) also affects the thermal comfort of humans [2]. Three common methods for minimizing computer overheating are: air cooling fans, thermal pastes, and passive cooling systems using PCMs. The air-cooling fan is the most widely used method for cooling computers. They are either installed inside computers/laptops or attached to computers’ bodies on the outside. Since air is a bad conductor of heat, this method has its own limitations for high-performing computers and laptops. Thermal pastes are popular for passive cooling of high-performing computers/laptops. These pastes are applied to processors and graphic cards that manage the peak temperature significantly due to their high thermal conductivity. However, pastes can be applied only in small quantities, which limits their thermal management. Moreover, pastes are difficult to apply, and their effectiveness degrades with time. Passive cooling systems using PCMs are less frequently used for cooling purposes on computers. Thermal pads of solid-to-solid PCMs have been used by Advance Micro Devices, Inc. (AMD), which were directly attached to processors, but it was reported that they had a negative effect on computers due to the blockage of natural air convection from the surface of the processors [3]. However, their low cost and ease of installation have attracted the research community to explore this option further.
In contrast to air cooling fans, which require external power, a passive thermal management system does not need extra power, with the additional advantage of silent operation [4]. Although both thermal pastes and thermal pads are types of passive thermal management systems, larger thicknesses of pads are possible (up to 20 mm thick) as opposed to a thin layer of thermal paste; a larger quantity of material provides cooling for a longer duration. The pads can be tailor-made in any geometry for a good fit and better surface contact. An ideal thermal pad material should be able to absorb infinite amounts of heat without undergoing any change. This apparently impossible feature can be achieved using PCMs. PCMs have very high latent heat, i.e., they have the ability to store large amounts of energy over a narrow temperature range. These materials absorb energy during the heating process as a phase change occurs (solid-to-liquid transformation) and release energy to the environment in the reversed-phase change during the cooling process [5–7]. Figure 1 shows that PCMs can be broadly classified as organic, inorganic, and binary eutectics, which are combinations of different organic and inorganic PCMs [8]. The selection of a particular class of PCM depends on the application, latent heat of fusion, melting point, safety, availability, and cost. Inorganic PCMs have superior latent heat, but their other properties, such as thermal conductivity, density, and specific heat capacity, are not consistent throughout the literature. Inorganic salts and salt hydrates have the potential to corrode encapsulation and storage materials, which makes them risky candidates for use in sensitive applications (high-performance laptop computers) [9]. One of the major problems with inorganic PCMs is supercooling, which decreases the stability and capacity for heat storage. Organic PCMs have specific and consistent properties that do not change significantly over time [10]. Organic PCMs are noncorrosive, which makes them suitable for use in sophisticated electronics without risking damage to the electronics. In addition, they have better thermal and cycling stability, which prolongs the life span of the cooling device [11].
PCMs are being used in buildings to increase thermal comfort, in solar panels for cooling, in the food-freezing industry, and in a wide range of other applications [11–15]. The concept of employing PCMs for cooling applications in electronics has been investigated in recent years [16–21]. Tan et al. [16] used heat storage units (HSUs) filled with n-eicosane (an organic PCM) for cooling mobile electronic devices. The high latent heat of n-eicosane absorbed the heat dissipated from the chips, and the chip temperature was maintained below the allowable service temperature of 50°C for 2 hours. The orientation of the HSU significantly affected the temperature distribution. Arshad et al. [17] used pin-fin heat sinks embedded with parrafin wax and n-eicosane for passive cooling systems of electronic devices. Kandasamy et al. [18, 19] investigated the effect of parameters such as the power input, orientation of the package, and melting and freezing times under cyclic steady conditions on the overall performance of PCM-based cooling devices. Based on the numerical and experimental studies, they concluded that the PCM based design is an excellent candidate for transient electronic cooling applications. Maxa et al. [20] used an organic PCM directly on a chip package to buffer thermal energy by suppressing the temperature peaks in electronic components during peak loads and electronic shorts and noted a peak temperature reduction of up to 22°C. Ahmed et al. [21] used PT-37 and n-eicosane for the thermal management of tablet PCs. They reported that thin thermal energy storage units are capable of reducing the rate of temperature increase during operation. At the maximum input, heat storage in the PCM resulted in a back-cover temperature reduction of 40°C. They also discovered that the use of PCMs encapsulated in aluminized laminate films was a practical solution for the thermal management of portable electronic devices.
Although organic PCMs are chemically and thermally stable, their low thermal conductivity limits their heat absorption capacity. However, these properties can be enhanced by the addition of nanomaterials (carbon nanotubes, graphene oxide, nickel nanorods, etc.) [22–24]. At peak performance, the temperature of the CPU rises up to 100°C, which elevates the temperature of the cover to 60°C. PCMs with melting points above the ambient temperature (even in hot summers) and below 60°C are suitable for cooling computers by utilizing their latent heat. Polyethylene glycol-1500 (PEG-1500), an organic PCM, has a melting range of 44–48°C and can fulfill these requirements. Additionally, when the temperature exceeds 48°C, the difference in temperature will provide effective cooling to the computer by increasing the rate of heat transfer. Considering all the aspects of material selection [25–29], PEG-1500 was used in this study. PEG-1500 was reinforced with GO nanoparticles to improve its thermal conductivity and thermal stability [30], which resulted in a higher heat transfer rate and rapid cooling. Thermal pads were developed by enclosing the GO/PEG-1500 composite in aluminum packs. The thermal pads provide effective cooling and manage the temperature by significantly lowering the peak temperature of the devices. An ambient temperature less than the melting (freezing) temperature of GO/PEG-1500 guarantees the recharging of the pads without any additional treatment.