Graphite Composite Phase Change Research Articles

Experimental and numerical study on thermal performance of Wood's alloy/expanded graphite composite phase change material for temperature control of electronic devices

The current investigation focuses on the feasibility of effectively managing the thermal conditions of electronic devices using Wood's alloy/expanded graphite composite phase change material (PCM). A thermal management system integrated with the composite PCM-based substrate is constructed and the thermal behaviors of this composite PCM under different testing conditions are studied through both experimental and numerical ways. The results indicate that the thermal performances of the composite PCM are dependent on input power density of the electronic chip, heat storage density and thermal conductivity of the PCM, and thickness of the PCM-based substrate. The temperature holding capability of the PCM can be obtained under high input power density. Under low input power density, the substrate only behaves as a heat spreader. Enhancing the heat storage density of the composite PCM is advantageous to obtain longer critical time (the required time for the chip to reach a critical temperature). Also, varying the thermal conductivity of the PCM from 14.9 to 59.6 W⋅m−1⋅K−1 leads to slight changes to the PCM's temperature control performance during the chip's working period under natural cooling conditions, whereas extending the thickness of the substrate can both lower the chip's equilibrium temperature and prolong the critical time.

Structure and thermal properties of expanded graphite/paraffin composite phase change material

ABSTRACTDifferent contents of expanded graphite (EG) composite phase change material (PCM) were prepared by the melt mixing method, taking paraffin as the PCM and EG as the supporting material. Phase compositions of EG, paraffin, and EG/paraffin composite were investigated using X-ray diffraction (XRD). Microstructures of EG and EG/paraffin composite PCMs with different EG contents were observed by a scanning electron microscope (SEM). Thermal properties, such as phase-transition temperature and latent heat of the materials, were determined by differential scanning calorimetry (DSC). Mass loss and thermal properties after 100 heating cycles were measured. The results show that physical absorption exists between paraffin and EG. EG is beneficial for the PCM composite to reduce leakage of paraffin, decrease the phase change temperature and latent heat, and strengthen the thermal stability. The solid–liquid phase change latent heat of materials is larger than that of the solid–solid one. The heating cycle has little effect on the phase-transition temperature and latent heat.

Experimental investigations of Alum/expanded graphite composite phase change material for thermal energy storage and its compatibility with metals

A novel composite phase change material(CPCM) were prepared with Aluminum potassium sulfate dodecahydrate(Alum, KAl(SO4)2·12H2O) as PCM and expanded graphite(EG) as nucleating agent and porous matrix for thermal conductivity enhancing. The detailed thermo-physical properties and thermal energy storage performance were studied. The DSC revealed that the melting temperature and latent heat of fusion of Alum/EG CPCM were 87.92 °C and 214.9J/g, respectively. The thermal conductivity of Alum/EG CPCM was improved from 0.497W·m−1·K−1 to 5.875W·m−1·K−1 due to the addition of EG, which can also be confirmed by the infrared thermal imager during charging/discharging process. The Alum/EG CPCM exhibited prominent chemical stability and thermal reliability before and after 500 thermal cycle tests. Furthermore, the corrosion of three metals and one metal alloy were studied and the gravimetric analysis and the results of element composition of four samples exhibited that the brass was the suitable materials for the container for long-term used while stainless steel 304L and aluminum were severely corroded. The results demonstrated that Alum/EG CPCM was a prospective candidate for thermal energy storage and accelerated the research on the Alum/EG heat storage system.

A shape-stabilized MgCl2·6H2O–Mg(NO3)2·6H2O/expanded graphite composite phase change material with high thermal conductivity and stability

Household water heating systems incorporating phase change materials (PCMs) can store solar thermal energy and reduce energy consumption. MgCl2·6H2O–Mg(NO3)2·6H2O eutectics have a melting point near 60 °C with a large energy storage density suitable for use in the thermal energy storage unit of a water heating system. However, the low thermal conductivity of the eutectic limits its heat extraction rate. In this study, we combine the eutectic and expanded graphite (EG) to prepare a high thermal conductivity composite PCM and study its cycling performance over 200 thermal cycles. The thermal conductivity of the composite PCM increased from 0.5 to 3.7 W m−1 K−1, ensuring faster heating and cooling. The images obtained from scanning electron microscopy and leakage testing showed that the maximum loading mass fraction of the eutectic in the EG reached 85 wt %. Infrared spectra and X-ray diffractometry patterns confirm that composites with compositions between those of EG and the eutectic were composed of the physical mixture. Thermogravimetric analysis indicated that the composite PCM lost less mass at 300 °C compared to the pure eutectic, and differential scanning calorimetry demonstrated that the composite PCM had a melting point of 52.3 °C and a small subcooling degree of 2.7 °C, compared to 56.6 and 18.4 °C for the pure eutectic, respectively. All these measurements demonstrated that the composite PCMs changed less after 200 thermal cycles than the pure eutectic, which had an observable stratification caused by phase separation. The composite PCM developed was thermally conductive, shape-stabilized, and more thermally stable during cycling than the pure eutectic.

Thermal properties and thermal conductivity enhancement of composite phase change material using sodium acetate trihydrate–urea/expanded graphite for radiant floor heating system

For heat exchanger used as latent heat storage system, heat storage capacity and thermal conductivity of phase change material are key indicators to determine its performance. In this paper, sodium acetate trihydrate-urea non-eutectic mixture as phase change material and expanded graphite as thermal conductivity enhancer, a sodium acetate trihydrate-urea/expanded graphite composite phase change material (CPCM) with both high latent heat and thermal conductivity for heat exchanger was developed by physical mixing method. The mass fractions of urea in the mixture and expanded graphite in CPCM were optimized. The crystalline phase and morphology of the obtained CPCM were characterized by X-ray diffraction and scanning electron microscope, and its thermal properties were investigated systematically. The results showed that the prepared CPCM containing 8% urea and 8% expanded graphite had a suitable phase change temperature (47.84 °C), high latent heat (223.1 kJ·kg−1), low supercooling degree (1.54 °C) as well as a satisfactory thermal conductivity (2.076 W·m−1·K−1). Moreover, the CPCM possessed a good shape stability and comparable thermal reliability. The results indicate that the obtained CPCM is an up-and-coming candidate for heat exchanger.

Numerical investigation on paraffin/expanded graphite composite phase change material based latent thermal energy storage system with double spiral coil tube

Latent thermal energy storage (LTES) by using phase change material (PCM) is a promising technology which can solve the temporal and geographical mismatch problems between energy supply and demand in utilization of solar thermal energy and industrial waste heat. However, until now, it is still challenging to provide LTES systems with high energy storage efficiency because of the low thermal conductivity of the PCM and suboptimum structure of heat exchanger. In this work, a novel LTES system with paraffin/expanded graphite (EG) composite PCM and double spiral coil tube as heat-exchange tube was constructed. The radius of the double spiral coil-shaped tube was determined by means of the quasi steady state method. And the effects of heat transfer fluid (HTF) inlet position and the ratio of the heat-exchange area between the inner and outer coil tube on the latent heat storage performance of this system were investigated. Compared with single coil tube, LTES system with double spiral coil tube could reduce the time to finish latent heat storage.

Heat Transfer Performance Study of Composite Phase Change Materials for Thermal Management

Latent heat storage has proved to be an effective means for thermal management for spacecraft due to its high storage capacity and small temperature variation from storage to retrieval. Although paraffin waxes offer an excellent source of thermal storage, the typically low thermal conductivity presents a significant challenge in the design of thermal management systems. The good physical and chemical properties of graphite and silicon carbide foam reveal the possibility of using them as a thermal conductivity enhancer for the phase change materials. A graphite or silicon carbide foam composite phase change material has a high effective conductivity and becomes one of the most interesting thermal storage techniques due to its advantage of simplicity and reliability. To simulate the heat transfer of silicon carbide and graphite foam composite phase change material, a finite volume technique is used to discretize the heat diffusion equation, and the two-temperature model is employed for the small-scale material, whereas the equivalent specific heat method is used for the large-scale material. The experimental setup is built up, and heat transfer experiments are performed to validate the proposed numerical method. Studies on the effects of the porosity on the equivalent thermal conductivity and the effects of the microstructure on the equivalent thermal conductivity are based on the numerical method.

Ca(NO3)2-NaNO3/expanded graphite composite as a novel shape-stable phase change material for mid- to high-temperature thermal energy storage

A novel shape-stable Ca(NO3)2-NaNO3/expanded graphite composite phase change material (PCM) for mid- to high-temperature TES was successfully synthesized by the developed impregnation and sintering two-step method. The low-cost binary salt Ca(NO3)2-NaNO3 was chosen as the PCM and expanded graphite (EG) was as the structural supporting material and thermal conductivity enhancer. The results showed that the binary eutectic nitrate and EG have good chemical compatibility, and the composite with 7 wt% EG can overcome the problems of cracks or liquid leakage during the phase change. Benefiting from the established better conductive passageway within the composites, the thermal conductivity of the composite with 7 wt% EG can be significantly increased by about 7.3 times in comparison with the binary eutectic nitrate. However, the measured phase change latent heats of the shape-stable phase change materials (SSPCMs) are markedly lower than the theoretical ones, meaning some losses of the latent heat during the material fabrication process. The developed SSPCM also showed little change of the phase change properties after 500 thermal cycles, indicating it has good thermal reliability for a long term period. Compared with other PCMs for mid- to high-temperature applications, this novel cost-effective composite SSPCM is very promising due to the wide operating temperature range and high thermal conductivity.

A polymer-coated calcium chloride hexahydrate/expanded graphite composite phase change material with enhanced thermal reliability and good applicability

A novel polymer-coated composite PCM was prepared by mixing the CaCl2·6H2O/EG composite particles with a photo-curing polymer, followed by irradiating the mixture under UV light to make the polymer cured. It is shown that the surfaces of the composite particles have been sealed by the polymer. And the melting and freezing enthalpy values of the obtained polymer-coated CaCl2·6H2O/EG composite PCM are as high as 113.2 and 114.1 J g−1, respectively, and its melting point is quite close to that of CaCl2·6H2O. After experiencing 500 heating-cooling cycles, polymer-coated CaCl2·6H2O/EG exhibits the decreases only by 1.68% and 0.52% in the enthalpies of melting and freezing, much less than 25.54% and 30.55% for CaCl2·6H2O/EG, respectively, revealing much enhanced thermal reliability. The applicability of the uncoated and polymer coated CaCl2·6H2O/EG composite PCMs was investigated by directly mixing them with gypsum powder and water, followed by forming into plasterboards. It is found that, the plasterboard containing polymer coated CaCl2·6H2O/EG not only exhibits superior thermal performance for delaying temperature rise over the one with CaCl2·6H2O/EG but also possesses much better thermal reliability. These good characteristics render the polymer-coated CaCl2·6H2O/EG composite PCM with great potentials in building energy conversation.

Hydrophilic Modification of Expanded Graphite to Prepare a High-Performance Composite Phase Change Block Containing a Hydrate Salt

A novel strategy for preparing hydrate salt/expanded graphite (EG) composite phase change materials (PCMs) with large latent heat capacity and high thermal conductivity is explored, which involves modifying EG with a surfactant, compressing the modified EG into a block, and immersing the block into a melted hydrate salt. TritonX-100 is employed to modify EG for improving its hydrophilicity. The block fabricated from the modified EG exhibits larger adsorptive capacity for a MgCl2·6H2O-NH4Al(SO4)2·12H2O eutectic as compared with that of the unmodified one. The obtained eutectic/modified EG composite block exhibits a melting temperature of 63.40 °C and a melting enthalpy of as high as 157.8 J/g. The thermal conductivity of the composite block is measured to 4.789 W/m·K, about 10 times that of the eutectic. The novel eutectic/modified EG composite block shows great potentials in solar utilization systems, and this work sheds light on the development of EG-based composite PCMs containing hydrate salts.

Improved thermal properties of stearyl alcohol/high density polyethylene/expanded graphite composite phase change materials for building thermal energy storage

Form–stable phase change materials (FSPCM) with high thermal energy storage capacity and thermal conductivity were manufactured by adding expanded graphite (EG) into stearyl alcohol (SAL) and high density polyethylene (HDPE) mixtures. In the composites, HDPE was used to prevent SAL leakage, and EG was not only a supporting material just like HDPE but also a thermal conductivity promoter. The influences of EG on the thermal properties of the SAL/HDPE phase change materials were investigated with a series of measuring means. Several lines of evidence showed that SAL and HDPE were mixed uniformly and EG was evenly dispersed in the SAL/HDPE composites. Approximately constant melting temperatures of FSPCM at around 57°C and high latent heat of at least 200kJ/kg were presented by the differential scanning calorimeter (DSC). It was found that the leakage rate of the SAL phase change materials decreased obviously when EG was added into the FSPCM. Furthermore, the thermal conductivity of FSPCM with 3% EG could increase up to 0.6698W(mK)−1 while thermal conductivity of FSPCM without EG was only 0.1966W(mK)−1. Finally, the FSPCM with EG could provide considerable thermal energy storage capacity and high thermal conductivity for latent heat storage.

A numerical study of building integrated with CaCl2·6H2O/expanded graphite composite phase change material

Integrating phase change material (PCM) into traditional building envelopes can decrease the building energy consumption and improve the indoor thermal comfort by enhancing the heat storage capability. In this work, CaCl2·6H2O/expanded graphite (EG) was employed to fabricate the PCM panels for building envelope as a low-cost PCM. In order to give full play to the role of CaCl2·6H2O/EG, its characteristic in structural insulated panel (SIP) was investigated numerically under different conditions. The enthalpy-porosity technique was used for modeling the solidification/melting process and it was validated by experiment. Temperature time lag, temperature decrement factor and energy saving rate were used to evaluate the dynamic thermal characteristics of the building envelope. The results showed that the density and thermal conductivity of PCM had little effect on the thermal performance of building envelope. However, the change of PCM panel thickness, building orientation, climate region and building construction would affect the phase change process of the PCM panels, which would have a certain influence on the room temperature. By analyzing the effect of CaCl2·6H2O/EG on the thermal performance of the building envelope under different influencing factors, it will provide effective guidance for the selection and use of PCM.

A novel process for preparing molten salt/expanded graphite composite phase change blocks with good uniformity and small volume expansion

Herein a novel process was explored for preparing molten salt/expanded graphite (EG) composite phase change material (PCM) blocks, which involves mixing a solid molten salt with EG thoroughly, compressing the mixture into a block with a designed shape and then heating the block to a temperature above the melting point of the molten salt followed by cooling. An MgCl2-KCl eutectic salt was used as the PCM, and its EG-based composite PCM block was prepared by this process, in which the optimal mass fraction of the eutectic was determined to be around 85%. The microstructure of the MgCl2-KCl/EG composite PCM block containing 85% the eutectic shows a uniform distribution of the molten salt. The composite PCM block has a melting point of 424.14°C and a solidification point of 418.39°C, and its latent heat values are 161.37J/g for melting and 160.28J/g for solidification. Compared with the eutectic salt, the composite PCM block exhibits a reduction in supercooling by 3.7°C and an enhancement in thermal conductivity by 11-fold. It has been verified that the composite PCM block possesses excellent thermal reliability. Furthermore, the composite PCM block has been compared with the one prepared by the conventional method (first adsorption and then compression). It is found that the composite PCM block fabricated by the novel process exhibits better uniformity and smaller volume expansion than the one obtained from the conventional method. The MgCl2-KCl/EG composite block shows great promise in high-temperature thermal energy storage systems, and this novel process is very suitable for preparing molten salt/EG composite PCM blocks.

Preparation and thermal property analysis of Wood’s alloy/expanded graphite composite as highly conductive form-stable phase change material for electronic thermal management

This paper reported a Wood’s alloy/expanded graphite (EG) composite phase change material (PCM) with high thermal conductivity and good form-stability. In this composite PCM, Wood’s alloy served as heat storage medium and EG acted as both heat transfer promoter and packaging material. The thermal properties of this composite PCM were investigated by means of differential scanning calorimetry (DSC) and transient plane source (TPS) method. The results showed that the phase change temperature of the composite was about 70.5 °C. The sensible and latent heat storage densities as well as thermal conductivity were influenced by the mass percentage of Wood’s alloy and the composite’s compacting density. The latent heat storage density and thermal conductivity could reach up to 113.1J·cm−3 and 65.0W·m−1·K−1. Appearance observation of the composite revealed that increasing the mass percentage of the alloy and decreasing the compacting density could help to keep the composite form-stable. These findings approved the potential of the Wood’s alloy/EG composite for use in thermal management of high power electronic devices.

Thermal Properties of Cement-Based Composites for Geothermal Energy Applications.

Geothermal energy piles are a quite recent renewable energy technique where geothermal energy in the foundation of a building is used to transport and store geothermal energy. In this paper, a structural–functional integrated cement-based composite, which can be used for energy piles, was developed using expanded graphite and graphite nanoplatelet-based composite phase change materials (CPCMs). Its mechanical properties, thermal-regulatory performance, and heat of hydration were evaluated. Test results showed that the compressive strength of GNP-Paraffin cement-based composites at 28 days was more than 25 MPa. The flexural strength and density of thermal energy storage cement paste composite decreased with increases in the percentage of CPCM in the cement paste. The infrared thermal image analysis results showed superior thermal control capability of cement based materials with CPCMs. Hence, the carbon-based CPCMs are promising thermal energy storage materials and can be used to improve the durability of energy piles.

Preparation and thermal properties of SAT-CMC-DSP/EG composite as phase change material

In this study, a series of SAT-CMC-DSP/EG (sodium acetate trihydrate-disodium hydrogen phosphate dodecahydrate-carboxyl methyl cellulose/expanded graphite) composite phase change materials (CPCMs) with different mass ratio of EG was prepared. The influence of EG addition on the thermal conductivity was examined using transient plane source method, thermal properties including melting temperature and latent heat were evaluated using differential scanning calorimetry (DSC), the latent heat storage and release time and the degree of supercooling were obtained using cooling curves method. The results showed that as a thermal conductivity enhancer, EG could improve the thermal conductivity of SAT effectively. At the same time, it brings some problems with the increase of EG mass fraction, such as lower enthalpy and higher subcooling degree. The SAT-CMC-DSP/EG with 3wt% EG was considered as the most suitable choice with the melting temperature, latent heat, thermal conductivity and degree of supercooling were 58.1°C, 210.7kJ/kg, 1.37W/(mK) and 2°C. The impact of external heating temperature on the composite PCM containing 3wt% EG was tested. The results showed that the heating temperature is should better not exceed 78°C. Furthermore, thermal cycling test results indicated that SAT-CMC-DSP/EG showed a good thermal reliability.

Experimental and numerical investigations on the thermal performance of building plane containing CaCl2·6H2O/expanded graphite composite phase change material

CaCl2·6H2O/expanded graphite (EG) composite phase change material (PCM) with melting and freezing points of 27.11°C and 21.67°C and corresponding latent heat values of 118.7J/g and 115.7J/g was employed to fabricate the PCM panels for building application. The thermal performance of the test room equipped with the PCM panels at each position was evaluated by placing the room in an artificial climatic chamber. Compared with the reference one without the PCM panels, the test room equipped with the PCM panels exhibited much lower maximum temperature, higher minimum temperature, and reduced temperature amplitude, revealing the function of the PCM panels for reducing indoor temperature fluctuation. Furthermore, the investigation on the effect of the position of the PCM panels indicated that, the more inside position the PCM panels were placed at, the better thermal performance the obtained test room had. Moreover, numerical modeling was carried out to study the thermal performance of the PCM panels containing the CaCl2·6H2O/EG composite. It was found that the results from the numerical modeling agreed well with those from the experimental investigations. Base on the numerical modeling, it was found that the optimum thickness of the PCM panel was about 8–10mm, and the thermal performance of the CaCl2·6H2O/EG panels was better than that of the RT27/EG. It is revealed that the CaCl2·6H2O/expanded graphite composite PCM shows great promise in building energy saving.

Structure and thermal properties of decanoic acid/expanded graphite composite phase change materials

Decanoic acid/expanded graphite composite phase change materials (DA/EG-PCMs) with high stability and excellent thermal conductivity were fabricated by blending expanded graphite (EG) and decanoic acid (DA). The structure, thermo-physical properties, and the formation mechanism of DA/EG-PCMs were investigated. The obtained results demonstrate that EG exhibits a network-like porous structure, which is superimposed of 10–50 μm thick graphite sheet. Therefore, DA can be effectively encapsulated through the binding between micropores and the surface adsorption of EG resulting in a relatively smaller DA/EG-PCMs particle with better dispersibility. In addition, adding EG into DA also increased both the thermal stability and the thermal conductivity while decreasing the charging and discharging time, which resulted in improved thermal efficiencies. Although adding EG can negatively influence the phase change behavior of DA, the temperature and enthalpy of phase change were still as high as 34.9 °C and 153.1 J g−1, respectively. Based on a combination of experimental results and a comprehensive analysis of the phase transformation kinetics, it is concluded that DA/EG-PCMs with 10 mass% EG with improved thermal properties can meet the requirements for efficient temperature control in low-to-medium environments.

Experimental Investigation on Thermal Management of Electric Vehicle Battery Module with Paraffin/Expanded Graphite Composite Phase Change Material

The temperature has to be controlled adequately to maintain the electric vehicles (EVs) within a safety range. Using paraffin as the heat dissipation source to control the temperature rise is developed. And the expanded graphite (EG) is applied to improve the thermal conductivity. In this study, the paraffin and EG composite phase change material (PCM) was prepared and characterized. And then, the composite PCM have been applied in the 42110 LiFePO4 battery module (48 V/10 Ah) for experimental research. Different discharge rate and pulse experiments were carried out at various working conditions, including room temperature (25°C), high temperature (35°C), and low temperature (−20°C). Furthermore, in order to obtain the practical loading test data, a battery pack with the similar specifications by 2S∗2P with PCM-based modules were installed in the EVs for various practical road experiments including the flat ground, 5°, 10°, and 20° slope. Testing results indicated that the PCM cooling system can control the peak temperature under 42°C and balance the maximum temperature difference within 5°C. Even in extreme high-discharge pulse current process, peak temperature can be controlled within 50°C. The aforementioned results exhibit that PCM cooling in battery thermal management has promising advantages over traditional air cooling.

Investigations on the thermal stability, long-term reliability of LiNO3/KCl – expanded graphite composite as industrial waste heat storage material and its corrosion properties with metals

The aim of this work was to investigate the properties of LiNO3/KCl-expanded graphite (EG) composite phase change material (PCM) concerning its long-term usage for industrial waste heat storage. Studies on the thermal stability and long-term reliability of this composite PCM as well as its compatibility with metals that commonly used in industries were conducted. By means of thermogravimetric analyzer (TGA) and differential scanning calorimeter (DSC), the initial thermal decomposition temperature of the composite PCM was found to be 315°C, and less than 1% and around 4% changes in phase change temperature and phase change latent heat, respectively, were obtained after repeated melting/solidification cycles. Gravimetric analysis of different metal specimens after enduring 500 thermal cycles in contact with the composite PCM revealed that stainless steel 304L and carbon steel C20 could be considered as more suitable containment materials than brass H68 for long-term use. The results of this work not only confirmed the great potential of LiNO3/KCl-EG composite PCM to be used as industrial waste heat storage medium, but also could facilitate experimental and numerical researches on the LiNO3/KCl-EG composite PCM-assisted latent heat storage systems in future.