CN104877641A - Method for low-cost quick preparation of paraffin/graphite phase-change composite material - Google Patents

Abstract

A method for low-cost quick preparation of a paraffin/graphite phase-change composite material includes: mixing natural flake graphite with graphite, putting the mixture into a preheated steel die, pressing under the pressure of 200-500kg/cm<2>, keeping the pressure for 5-15min, and stripping timely to obtain the paraffin/graphite phase-change composite material, wherein preheating temperature of the steel die is 42-90 DEG C. The method for low-cost quick preparation of the paraffin/graphite phase-change composite material has the advantages of low cost, avoidance of pollutants, excellent heat conducting performance and high heat storage capacity.

Description

A rapid and low-cost method for preparing paraffin/graphite phase change composites

technical field

The invention belongs to a low-cost method for preparing a paraffin/graphite phase-change composite material with excellent thermal conductivity and strong heat storage capacity.

Background technique

In the process of thermal energy storage and utilization, there are often contradictions between the supply time and space mismatch, such as the intermittent nature of solar energy, the peak-valley difference of electric load, the heat dissipation of high-power electronic cooling devices that work periodically, and the utilization of industrial waste heat, etc. . Phase change energy storage materials realize energy storage and utilization by absorbing or releasing a large amount of heat when the material undergoes a phase change, which can effectively solve the contradiction between energy supply and demand in terms of time and space. Phase change energy storage materials can be divided into four categories according to the phase change mode: solid-liquid phase transition, solid-solid phase transition, liquid-gas phase transition and solid-gas phase transition. The latter two types of materials have a large amount of gas generation and large volume changes during the phase transition process, and are rarely used in practical applications. Solid-solid phase change materials have the disadvantages of high phase transition temperature, low phase transition enthalpy and high cost, and their applications are also greatly limited. Solid-liquid phase change materials are the most widely used phase change materials at home and abroad because of their low cost, large phase change latent heat, wide phase change temperature range, and no supercooling or precipitation. However, the phase-change energy storage materials themselves have the problems of low thermal conductivity (generally below 1W/m.K) and poor heat transfer performance, which will inevitably affect the energy storage and discharge rate. Therefore, it is necessary to combine phase change energy storage materials with materials with excellent thermal conductivity, so as to achieve the purpose of enhancing heat transfer of phase change energy storage materials.

For this reason, many scientific and technological workers at home and abroad have carried out a lot of research work on the composite of solid-liquid phase change materials and materials with excellent thermal conductivity. It is a relatively simple and practical method to add metal particles or powders with better thermal conductivity (such as Ag, Al and Cu powder) into phase change materials to improve the thermal conductivity of phase change materials. After adding Al powder to the phase change energy storage material in solar water heaters, American scientists found that when the mass content of Al powder reaches more than 50%, the heat conduction efficiency increases by nearly 4 times. Zeng et al. systematically investigated the effect of the addition of Ag particles on the thermal conductivity of tetradecane, a phase change material. Studies have shown that with the increase of the mass of Ag particles, the thermal conductivity of the composite material increases, but the latent heat of the material decreases; in addition, because the metal generally has a higher density, the weight of the entire heat storage system increases and the practicability becomes worse. A new type of material developed in recent years is to combine porous materials such as high thermal conductivity metal foam, graphite foam and expanded graphite with phase change materials to improve the thermal conductivity and heat transfer efficiency of phase change materials. However, metal foams are prone to corrosion, high density, and poor compatibility with phase change materials, which limit their applications. Porous carbon materials with light weight and good thermal conductivity, such as graphite foam and expanded graphite, have many advantages such as high thermal conductivity, low density, strong chemical stability, and good compatibility with phase change materials. They are ideal storage materials for phase change materials. carrier. Zhong Yajuan et al. (Heat transfer enhancement of paraffin wax using graphite foam for thermal energy storage. Zhong Y J, et al. Solar Energy Materials & Solar Cells, 2010, 94:1011-1014) used graphite foam to absorb paraffin wax to prepare paraffin/graphite Foam Phase Change Composites. The results show that the pore structure and thermal conductivity of graphite foam are important factors affecting the thermal properties of the final composite. Compared with the thermal conductivity of paraffin (0.3W/m.K), the thermal conductivity of paraffin/graphite foam composite material has increased nearly 50 times, up to 14.6W/m.K. However, graphite foam is generally prepared through processes such as asphalt foaming, high-pressure carbonization, and high-temperature graphitization (above 2500 ° C), which not only requires high equipment (high temperature and high pressure), long production cycle, and high energy consumption (high cost). widely used. Zhong Yajuan et al. (Heat transfer enhancement of paraffin wax using expanded naturegraphite for thermal energy storage. Carbon, 2010, 48:300-304) also used compressible expanded graphite (CENG) as a storage carrier, and adopted vacuum impregnation Composite with phase change material paraffin to prepare paraffin/CENG composite. Compared with the thermal conductivity of paraffin, the thermal conductivity of paraffin/CENG composite material has increased by more than 25 times, but due to the low thermal conductivity of CENG itself, the thermal conductivity of the final composite material is limited to below 50W/m.K, which is difficult to meet in some Requirements for use in the field of special heat dissipation. In addition, in the process of CENG preparation, the natural graphite used as raw material must go through processes such as acidification, water washing and high-temperature expansion (about 900 ° C), which not only has a long production cycle but also produces a large amount of waste gas and waste water, causing serious pollution to the environment.

Contents of the invention

The purpose of the present invention is to provide a method for preparing a paraffin/graphite phase change composite material with low cost, no pollutant generation, excellent thermal conductivity and high heat storage capacity.

The raw materials used in the preparation process of the material of the present invention are natural flake graphite and paraffin wax, wherein the natural flake graphite is a heat conducting medium, and the paraffin wax is both a phase change carrier and a molding binder. After the above two raw materials are evenly mixed, the preparation of the paraffin/graphite phase change composite material is completed through a hot pressing molding process. The paraffin/graphite phase-change composite material that the present invention proposes, its preparation method comprises the steps:

(1) 30-45% by weight of natural flake graphite is mixed with 55-70% of paraffin wax. After preheating in the heater to 92-140°C, add it into the melted paraffin wax liquid at constant temperature according to the required proportion, and continue to mix with mechanical stirring for 10-30min;

(2) After the mixing is completed, the mixed material is cooled to 42-90°C, and then it is put into a preheated steel mold, pressed and formed under a pressure of 200-500kg/ ^cm2 and kept for 5-15 minutes; Steel mold preheating temperature is 42-90°C;

(3) After the pressure holding is completed, the mold is released while it is hot to obtain a paraffin/graphite phase change composite material.

The above natural flake graphite has an average particle size of 75-380 μm and a carbon content of 96-99 wt.%.

The paraffin wax mentioned above has a melting point of 47-95°C, a molecular weight of 254-506, a molecular formula of CnH _2n+2 where n=18-36, and a heat of fusion (latent heat) of 140-280 J/g.

The composite material of the present invention has both the excellent thermal conductivity of graphite materials and the high heat storage and heat release capabilities of phase change energy storage materials, thereby greatly improving its heat exchange efficiency in energy storage systems and improving heat dissipation in high-power electronics. There will be broad application prospects in the field.

The main advantages of the present invention are as follows:

(1) Short production cycle: In the preparation process of the material of the present invention, it is only necessary to uniformly mix the heated and melted phase-change paraffin and natural flake graphite in proportion, and then the paraffin/graphite phase-change composite material can be obtained by compression molding, and the production cycle is greatly improved. shorten.

(2) Low production cost: In the production process of the above-mentioned paraffin/graphite foam phase change composite material and paraffin/CENG composite material, high temperature graphitization (above 2500°C) and high temperature expansion (850～1050°C) treatment are required respectively ; and the above-mentioned composite material relies on high temperature and high pressure to impregnate and infiltrate the melted phase-change paraffin into the pores of the graphite material using a vacuum-pressure process, which not only consumes a lot of energy, but also requires relatively high equipment requirements, making the production cost larger. However, in the preparation process of the composite material of the present invention, the material preparation can be completed without high-temperature sintering and high-pressure impregnation (the highest temperature is only 140° C.), so the energy consumption is small, and the preparation cost is also greatly reduced.

(3) No environmental pollution: For paraffin/graphite foam phase change composite materials, since the raw material used is asphalt, a large amount of hydrocarbons decompose, volatilize and escape during high temperature treatment, causing environmental pollution; while in the production of paraffin wax/CENG composite materials During the process, a large amount of waste acid and waste gas will be produced, which will also pollute the environment. During the production and preparation process of the composite material in the present invention, the natural flake graphite powder and phase-change paraffin used as raw materials have no volatile matter, so no pollutants will be generated.

(4) Excellent thermal conductivity of composite materials: The thermal conductivity of any type of graphite material depends on the degree of orientation of the internal microcrystalline structure. The higher the degree of alignment of the graphite microcrystalline structure, the better the thermal conductivity of the material. During the preparation of paraffin/CENG composites, the natural flake graphite used as raw material is acidified and expanded at high temperature, and the volume expands to more than 150 times, and the orientation of graphite crystallites inside is destroyed, showing a turbostratic structure. Although the degree of crystallite orientation will be improved again during the subsequent rolling process, it is impossible to return to the state of the original natural flake graphite, so the thermal conductivity of the final composite is relatively low. However, in the process of preparing composite materials using the process of the present invention, the inherent highly oriented structure of the natural flake graphite particles used as raw materials is not damaged, and is maintained from the beginning to the end; Orientation occurs along the direction perpendicular to the pressing direction, so the thermal conductivity of the prepared paraffin/graphite phase change composite material is also high, which can greatly improve its heat transfer efficiency in the energy storage system.

Detailed ways

Example 1

First put 60% paraffin wax into a steel mixing tank and heat it to melt, and then keep the temperature at 95°C; the paraffin wax used has a melting point of 50°C, a molecular weight of 282, and a molecular formula of C ₂₀ H ₄₂ . (Latent heat) was 167 J/g. After the natural flake graphite is preheated to 95°C in a heater, it is added to the above-mentioned molten paraffin wax liquid at 40% by weight, and continuously stirred and mixed for 15 minutes, wherein the average particle size of the natural flake graphite used is 350 μm, carbon The content is 98.6wt.%. After mixing, the mixed material is cooled to 45°C, then put it into a preheated steel mold, and then perform compression molding under a pressure of 300kg/ ^cm2 and keep the pressure for 10min, wherein the steel mold preheating temperature is 45 ℃. After the holding pressure is completed and the mold is released while it is hot, the preparation of the paraffin/graphite phase change composite material is completed, and its basic physical properties are shown in Table 1.

Example 2

Put 66% paraffin wax into a steel mixing tank and heat it to melt, then keep the temperature to 117°C; the paraffin wax used has a melting point of ₇₂ °C, a molecular weight of 366, a molecular formula of _C26H54 , and its heat of fusion ( Latent heat) is 194J/g. After the natural flake graphite was preheated to 117°C in a heater, it was added to the above-mentioned molten paraffin wax liquid at 34% by weight, and continued to stir and mix for 20min, wherein the average particle size of the natural flake graphite used was 264 μm, carbon The content is 96.8wt.%. After the mixing is completed, the mixed material is cooled to 67°C, and then it is put into a preheated steel mold, and then press-molded under a pressure of 360kg/cm ² and held for 8 minutes, wherein the preheating temperature of the steel mold is 67 ℃. After the holding pressure is completed and the mold is released while it is hot, the preparation of the paraffin/graphite phase change composite material is completed, and its basic physical properties are shown in Table 1.

Example 3

First put 75% paraffin wax into a steel mixing tank and heat it to melt, and then keep the temperature at 130°C; the paraffin wax used has a melting point of 85°C, a molecular weight of 436, and a molecular formula of C ₃₁ H ₆₄ . (Latent heat) was 218 J/g. After the natural flake graphite is preheated to 130°C in a heater, it is added to the above-mentioned molten paraffin liquid at 25% by weight, and continuously stirred and mixed for 6 minutes, wherein the average particle size of the natural flake graphite used is 206 μm, carbon The content is 99wt.%. After mixing, the mixed material is cooled to 80°C, then put it into a preheated steel mould, and then carry out compression molding under a pressure of 200kg/ ^cm2 and keep the pressure for 5min, wherein the preheating temperature of the steel mold is 80 ℃. After the holding pressure is completed and the mold is released while it is hot, the preparation of the paraffin/graphite phase change composite material is completed, and its basic physical properties are shown in Table 1.

Example 4

First put 75% paraffin wax into a steel mixing tank and heat it to melt, and then keep the temperature at 137°C; the paraffin wax used has a melting point of 92°C, a molecular weight of 448, and a molecular formula of C ₃₄ H ₄₀ . (Latent heat) was 249 J/g. After the natural flake graphite was preheated to 137°C in a heater, it was added to the above-mentioned molten paraffin wax liquid at 25% by weight, and continued to stir and mix for 5 minutes, wherein the average particle size of the natural flake graphite used was 264 μm, carbon The content is 99wt.%. After the mixing is completed, the mixed material is cooled to 87°C, and then it is put into a preheated steel mold, and then press-molded under a pressure of 240kg/cm ² and held for 7 minutes, wherein the preheating temperature of the steel mold is 87 ℃. After the holding pressure is completed and the mold is released while it is hot, the preparation of the paraffin/graphite phase change composite material is completed, and its basic physical properties are shown in Table 1.

Example 5

First put 55% paraffin wax into a steel mixing tank and heat it to melt, then keep the temperature at 97°C; the paraffin wax used has a melting point of 52°C, a molecular weight of 296, a molecular formula of C ₂₁ H ₄₄ , and its heat of fusion (Latent heat) was 175 J/g. After the natural flake graphite is preheated to 97°C in a heater, it is added to the above-mentioned molten paraffin wax liquid at 45% by weight, and continuously stirred and mixed for 20min, wherein the average particle size of the natural flake graphite used is 280 μm, carbon The content is 96.5wt.%. After the mixing is completed, the mixed material is cooled to 47°C, and then it is put into a preheated steel mold, and then press-molded under a pressure of 350kg/ ^cm2 and kept for 10 minutes, wherein the preheating temperature of the steel mold is 47 ℃. After the holding pressure is completed and the mold is released while it is hot, the preparation of the paraffin/graphite phase change composite material is completed, and its basic physical properties are shown in Table 1.

Example 6

First put 55% paraffin wax into a steel mixing tank and heat it to melt, then keep the temperature at 97°C; the paraffin wax used has a melting point of 52°C, a molecular weight of 296, a molecular formula of C ₂₁ H ₄₄ , and its heat of fusion (Latent heat) was 175 J/g. After the natural flake graphite is preheated to 97°C in a heater, it is added to the above-mentioned molten paraffin wax liquid at 45% by weight, and continuously stirred and mixed for 20min, wherein the average particle size of the natural flake graphite used is 80 μm, carbon The content is 97wt.%. After the mixing is completed, the mixed material is cooled to 47°C, and then it is put into a preheated steel mold, and then press-molded under a pressure of 350kg/ ^cm2 and kept for 10 minutes, wherein the preheating temperature of the steel mold is 47 ℃. After the holding pressure is completed and the mold is released while it is hot, the preparation of the paraffin/graphite phase change composite material is completed, and its basic physical properties are shown in Table 1.

Example 7

First put 65% paraffin wax into a steel mixing tank and heat it to melt, then keep the temperature at 103°C; the paraffin wax used has a melting point of 58°C, a molecular weight of 310, a molecular formula of C ₂₂ H ₄₆ , and its heat of fusion (Latent heat) was 184 J/g. After the natural flake graphite is preheated to 103°C in a heater, it is added to the above-mentioned molten paraffin wax liquid at 35% by weight, and continuously stirred and mixed for 20min, wherein the average particle size of the natural flake graphite used is 106 μm, carbon The content is 97.5wt.%. After the mixing is completed, the mixed material is cooled to 53°C, and then it is put into a preheated steel mold, and then pressed and formed under a pressure of 300kg/ ^cm2 and kept for 10 minutes, wherein the steel mold is preheated at a temperature of 53 ℃. After the holding pressure is completed and the mold is released while it is hot, the preparation of the paraffin/graphite phase change composite material is completed, and its basic physical properties are shown in Table 1.

Example 8

First put 70% paraffin wax into a steel mixing tank and heat it to melt, then keep the temperature at 112°C; the paraffin wax used has a melting point of 67°C, a molecular weight of 338, a molecular formula of C ₂₄ H ₅₀ , and its heat of fusion (Latent heat) was 197 J/g. After the natural flake graphite is preheated to 103°C in a heater, it is added to the above-mentioned molten paraffin liquid at 30% by weight, and continuously stirred and mixed for 16 minutes, wherein the average particle size of the natural flake graphite used is 142 μm, carbon The content is 98.5wt.%. After mixing, the mixed material is cooled to 62°C, then put it into a preheated steel mold, and then carry out compression molding under a pressure of 250kg/ ^cm2 and keep the pressure for 10min, wherein the steel mold preheating temperature is 62 ℃. After the holding pressure is completed and the mold is released while it is hot, the preparation of the paraffin/graphite phase change composite material is completed, and its basic physical properties are shown in Table 1.

Table 1 Basic physical performance parameters of materials

Remarks: ∥: parallel to the direction of the graphite layer; ⊥: perpendicular to the direction of the graphite layer.

Claims (3)

1. a method for preparing paraffin/graphite phase-change composite material fast and low-cost, is characterized in that comprising the steps: (1) Mix natural flake graphite with a weight percentage of 30-45% and 55-70% paraffin wax. During the mixing process, first heat and melt the paraffin wax into a liquid, and keep the temperature at 92-140°C. After preheating in the heater to 92-140°C, add it to the paraffin wax liquid at constant temperature after melting according to the required proportion, and continue to mix with mechanical stirring for 10-30min; (2) After mixing, cool the mixed material to 42-90°C, then put it into a preheated steel mould, press and form it under a pressure of 200-500kg/ ^cm2 and keep the pressure for 5-15min; Steel mold preheating temperature is 42-90°C; After the pressure holding is completed, the mold is released while it is hot to obtain a paraffin/graphite phase change composite material. 2. a kind of method for preparing paraffin/graphite phase-change composite material fast and low-cost as claimed in claim 1, is characterized in that the average particle size of described natural flake graphite is 75-380 μ m, and carbon content is 96-99wt%. 3. A method for preparing paraffin/graphite phase-change composites quickly and at low cost as claimed in claim 1, characterized in that the melting point of the paraffin is 47-95°C, the molecular weight is 254-506, and the molecular formula is CnH _{2n+ 2} , where n=18-36, its heat of fusion is 140-280J/g.