TECHNICAL FIELD

The present invention relates to an expanded graphite-phase change material composite that can be integrated into phone cases, which prevents overheating and hence the technical problems associated with this overheating affecting safe and long-term use of smart mobile phones.

PRIOR ART

Active and passive cooling methods are used to prevent overheating of electronic devices. These methods provide thermal protection by using a combination of suitable material composition and heat transfer techniques to keep the device temperature at an acceptable level. The necessity of using smaller electronic components in a small volume in mobile electronic devices increases the danger of overheating. An effective thermal protection system with high reliability is required to reduce heat radiation from smart phones that come into contact with the user's skin.

Fan system based on air forced convection is an active method widely used in cooling electronic devices. This method is not suitable for use in mobile devices as it requires large equipment such as a fan.

In current smart mobile phone applications, passive cooling is generally used to cool the processor (CPU) of the device with its geometric designs that will enable the selection of suitable materials with high thermal conductivity and heat conduction. However, it is known that these applications do not show enough performance in smart mobile phones whose usage is increasing day by day. Applications installed on smart mobile phones requiring excessive power use cause mobile phones to overheat.

The application of heat pipes in the cooling system of smart mobile phones has been tried in recent researches. The heat pipe consists of a liquid (such as water, alcohol) and a wick inside a thin metal pipe that is closed on both sides. When heat is applied to one end of the heat pipe, the liquid inside evaporates and rises towards the top of the pipe. When it comes to the cold upper region, it condenses into liquid and returns to the lower part of the pipe with the capillary effect of the wick. This process continues as long as there is a temperature difference between the two ends. Thus, the system is cooled by removing the heat from the region where it is more intense and spreading it over a wide area. There are difficulties in developing micro-sized heat pipes with appropriate size and geometry in smart phone cooling application.

Recently used and researched method for thermal protection of electronic devices and smart mobile phones is passive cooling with solid-liquid organic phasechanging material (PCM) with high melting-freezing latent heat. The melting temperature of PCM is selected in accordance with the critical temperature specified for the electronic device. Conveniently packaged and placed around the electronic device, PCM can effectively store heat emitted from electronic components during phase change from solid to liquid. Liquid PCM re-solidifies and turns into a reusable form by spreading heat to the environment when the electronic device is not operating.

Since phase change occurs isothermally at the melting temperature of the PCM, the temperature of electronic devices can be maintained at constant temperature until the PCM is completely melted. Thermal protection with PCM occurs in three stages.

In the first stage, the solid PCM heats from its initial temperature to its melting temperature with the heat emitted by the electronic device. Thus, the heat emitted by the electronic device is absorbed by the PCM and the thermal protection is started.

In the second stage, solid PCM changes phase at constant melting temperature. At this stage, the melting-freezing heat of PCM is stored as latent heat. Thus, thermal protection is completed by keeping the electronic device isothermal at its melting temperature.

In the third stage, when the ambient temperature decreases, the PCM freezes and becomes solid again.

For an effective thermal protection, PCM integrated into the smart mobile phone must quickly store the heat in the 1st and 2nd stages.

BRIEF DESCRIPTION OF THE INVENTION:

Unlike the previous techniques, our invention aims to provide passive cooling of the device easily by designing PCM-expanded graphite (EG) composite (III) with improved thermal conductivity to be integrated not into the interior of the smart phone, but either on its outer layer or in the case.

Thermal Energy Storage (TES) has a wide range of applications (chemical industry, food industry, district heating, central hot water, thermal comfort, electricity generation with concentrated solar power plants, etc.) that can provide heating, cooling and thermal protection. Three different methods are used for TES: sensible heat, latent heat and thermochemical.

In the sensible heat storage method, heat energy released as a result of the change in temperature, depending on the specific heat of the heat storage material, is stored. This method is widely used today. The reasons for this are; the fact that sensible heat storage materials are abundant and cheap, and they are very easy to integrate into systems.

Latent heat storage includes solid-liquid, solid-solid, liquid-gas and solid-gas conversions of the phase-changing material. Liquid-gas conversion is not preferred due to problems with pressure difference. During isothermal phase change, higher capacity and smaller volume of heat is stored than sensible heat. Since PCMs used as heat storage materials have low thermal conductivity, they should be supported with additives in applications requiring rapid storage. Metals, nano-particles and carbon derivatives can be used as additives for this purpose.

Thermochemical storage takes advantage of a reversible chemical reaction or sorption transformations. In the reversible reaction, heat is stored in the endothermic direction, while heat is recovered in the exothermic direction. Sorption systems include adsorption and absorption. Adsorption involves fixing or capturing a reactive gas on a solid surface. The heat storage capacity of the thermochemical method is the highest compared to others, but its development has not been completed yet.

Among TES methods, latent heat storage is used in our invention. The thermal conductivity of the PCMs used in this method should be increased in order to prevent smart mobile phones from overheating. The PCM composite we developed for this purpose is transformed from solid form to liquid form and stores the heat emitted during heavy use of the smart phone. The developed composite structure prevents the liquid PCM from spreading around and keeps it in its matrix structure. Whereas during the cooling of the device, the stored heat is released from the composite material and PCM turns into solid form again. Thus, the invention will be available repeatedly while the smart phone is in operation. FIGURE LIST

Figure 1. Schematic representation of the PCM-EG composite smart cell phone case design (showing the PCM-EG composite and the area where PCM-EG composite will be integrated into the case).

Figure 2. Application of the developed PCM-EG composite to the smart mobile phone (demonstration of the use of the PCM-EG composite in the mobile phone after its integration into the case).

Figure 3. Thermal protection performance of the developed PCM-EG composite in smart mobile phones.

DESCRIPTION OF THE NUMBERS USED IN FIGURES:

I. Polyethylene coating

II. Adhesive area (back of polyethylene coating)

III. PCM-EG composite

IV. Entire heat storage invention (size 5X5 cm)

V. Phone case

VI. The area where the invention will be applied to prevent overheating

VII. Smart mobile phone

DETAILED DESCRIPTION OF THE INVENTION

Smart mobile phones are widely used with the opportunities they provide to the consumer through various applications. In the rapidly developing smart mobile phone models, higher performance chips, graphics cards and batteries are used to meet consumer demands. The resistance to electric current during the operation of these components causes a significant heat generation and temperature increase. Increasing the performance of the components further increases the heat flow per unit area. Failure to control heat flow may result in a decrease in device performance; it may even cause breakage, material delamination, burning and explosion.

Today, when the use of smart mobile phones is increasing rapidly, overheating is observed also with the effect of the applications used in these devices and climatic conditions. In our literature survey, it is seen that electronic devices are actively cooled with large fans or passively cooled by the electronic components of the phone with suitable materials with high thermal conductivity and geometry designs that will enhance heat conduction. While the use of large fan systems in mobile devices is not considered appropriate due to its size, electronic cooling systems are insufficient for cooling smart mobile phones.

It is reported that the temperature range should not exceed 37-43 °C for safe and long-lasting use of smart mobile phones. In our invention, PCM composite has been developed to prevent overheating in smart mobile phones (VII). Paraffin with a melting range of 42-44 °C was chosen as PCM. Paraffin is preferred due to its thermal and chemical stability during phase change and its non-corrosive effect. Thermal conductivity values of paraffin at the level of 0.2 W I mK are insufficient in cases requiring rapid heat removal, such as smart mobile phones (VII). For this reason, PCM composite is prepared and used with components with high thermal conductivity. For this purpose, graphite with thermal conductivity 12.5 W/mK is used in PCM applications by expanding it with thermal application and increasing its thermal conductivity to 32.2 W/mK. Thermal conductivity value of PCM-expanded graphite (EG) composites (III) can be increased at least 10 times depending on the amount of EG. In our invention, PCM composites with 5% and 10% EG additive by weight were prepared. EG was obtained by expanding graphite by keeping it at 800 °C for 60 seconds. The prepared EG and paraffin in liquid form were prepared in the determined proportions (95% PCM for 5% EG by weight, 90% PCM for 10% EG by weight) and mixed in the sonicator for 4 hours. Afterwards, the Paraffin-EG composites obtained was dried in the oven. It has been observed that paraffin-EG composites whose thermal properties are determined by differential scanning calorimetry (DSC) given in Table 1 have appropriate melting-freezing temperatures and latent heat (AH).

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Table 1. DSC Results of Composites and PCM When the PCM is integrated into the designed smart phone case (V) in its pure form, leakage or shape deformities can be encountered. A more stable and leak-proof composite was prepared to eliminate this problem. When graphite is expanded, its porosity increases. At the end of sonication applied for 4 hours, the leakage of paraffin from the composite structure obtained by diffusing into the EG pores was prevented. Thus, thermal conductivity of paraffin used as PCM was increased and phase change was achieved without leakage. Obtaining high thermal conductivity is important in terms of storing heat quickly while temperature of the device rises. Looking at the thermal conductivity values given for three separate measurements and their averages in Table 2, it is seen that the thermal conductivity of the PCM-composite is increased by 13 to 21 times compared to paraffin, depending on the amount of EG.

Table 2. Thermal conductivity of the materials and the composites used

The composite materials obtained were prepared in 2 mm thickness and packaged in 5X5 cm2 with packaging material produced from polyethylene. Package dimensions can be sized according to the mobile phone model to be used. The packaged PCM-EG composite (III) material is integrated into the phone case (V) in a manner to come right on top of the CPU part of the mobile phone. Temperature variations were measured with thermocouple sensors to determine the overheating performance.

First of all, the temperature data of the mobile phone at room conditions was measured and recorded by a data-logger during the heating caused by heavy use with an empty case without adding PCM-composite. Then, under the same conditions, temperature measurements were taken by integrating PCM-5 %EG and PCM-10%EG composites into the case, respectively. Figure 3 shows the measurement results with blank, PCM-5%EG and PCM-10%EG composite material. These results show that when no PCM composite is used in the phone case (V), the temperature of the smart mobile phone (VII) reaches 40 °C after 2500 seconds. In the measurements taken after the integration of PCM-5 %GG composite to the phone case (V), it is seen that temperature reaches approximately to 34 ° C and approximately to 37 °C with the use of PCM-10 %EG composite at the end of the same period. Based on these results, the integration of the PCM-EG composite (III) in the 5X5 cm2 cell phone case (V), it is seen that the temperature was reduced by 6 °C in the PCM-5 %EG composite and 4 °C in the PCM-10 %EG composite respectively.

It was determined in the laboratory measurements that the PCM-EG packages developed in this study provide sufficient thermal protection for the smart mobile phone against overheating. Within the framework of the method created in this study, the type of PCM used when preparing a package containing an PCM composite, the type and amount of the additive used in composite preparation, the type of plastic used in the packaging and the dimensions of the prepared package may vary. Packages containing the prepared PCM composite can be easily added to the smart mobile phone case (V) and used repeatedly. The application of the packages containing the developed PCM-EG composite (III) is shown schematically in Figure 1. These composite materials, which prevent heating in smart mobile phones (VII) or all mobile devices as a result of the integration of the device into the case without damaging the electronic components, provide advantages for the safe and more economical use of smart mobile phones by consumers. In the recommended daily usage, the 5X5 cm2 (can be sized according to the mobile phone model to be used) polyethylene coating (I) package containing the PCM composite material obtained can be affixed to the mobile phone case (V) and reused with the adhesive.