Oxalic acid dihydrate-boric acid eutectic phase change composite material as well as preparation method and application thereof

Abstract

The invention discloses an oxalic acid dihydrate-boric acid eutectic phase change composite material as well as a preparation method and application thereof. According to the oxalic acid dihydrate-boric acid eutectic phase change composite material, oxalic acid dihydrate and boric acid are used as eutectic phase change material components, fumed silica is used as a porous adsorption carrier, expanded graphite is used as a conductive filler, and compounding is achieved through a melt blending method. According to the material, the high specific surface area and mesoporous structure of fumed silica are utilized, eutectic phase change material components are effectively adsorbed into pore channels of the fumed silica, expanded graphite serves as a lamellar heat conduction enhancing component, a continuous heat conduction channel is constructed in the material, and the thermal response capacity and structural stability of the oxalic acid dihydrate-boric acid eutectic phase change composite material are improved. By regulating and controlling the proportion of all the components, the oxalic acid dihydrate-boric acid eutectic phase change composite material keeps higher phase change latent heat, and meanwhile, the morphological stability and the cycling stability are remarkably improved. The material is simple in preparation process and can be widely applied to the field of medium-high temperature heat storage and electro-thermal conversion management.

Description

Technical Field

[0001] This invention relates to the field of phase change energy storage materials technology, specifically to a dihydrate oxalic acid-boric acid eutectic phase change composite material, its preparation method, and its application. Background Technology

[0002] With the adjustment of energy structure and the continuous increase in the proportion of renewable energy utilization, the efficient storage and regulation of thermal energy has become an important research direction in the field of energy utilization. Phase change thermal energy storage technology, due to its ability to absorb or release large amounts of heat in a near-isothermal manner during phase change, has advantages such as high thermal density and stable operation, and is widely used in solar thermal utilization, industrial waste heat recovery, building energy conservation, and thermal management systems. Among them, mid-temperature phase change thermal energy storage materials (approximately 80–120℃) have significant application value in practical engineering, as they can meet the operating temperature requirements of various industrial and civil systems while taking into account both system safety and material stability.

[0003] In recent years, with the rapid development of electric heating technology and electrothermal conversion systems, electrically driven thermal management and heat storage methods have gradually attracted attention. In devices such as electric heating energy storage devices, electrically driven thermal management systems, and some energy storage and temperature control devices, the electrothermal conversion unit typically converts electrical energy directly into heat energy, and then stores and slowly releases the heat through heat storage materials. Existing research shows that in most electrothermal conversion applications, the system operating temperature is usually concentrated in the medium temperature range, especially in the 80–120℃ range, where it has a high application frequency. Within this temperature range, if the phase change temperature of the matching heat storage material is too low, it will be difficult to fully utilize its heat storage function; while if the phase change temperature is too high, it will lead to a decrease in electrothermal conversion efficiency and an increase in system energy consumption. Therefore, developing phase change thermal storage materials with phase change temperatures that match the electrothermal conversion operating range, high thermal storage capacity, and stable operation is of great significance for improving the overall performance of electrothermal conversion systems (Xiao Q, Xu Y, Li X, et al. Enhanced solar-thermal and electro-thermal storage performance of solid-solid composite phase change material[J]. Composites Communications, 2024, 45:101818.).

[0004] Currently, common phase change thermal storage materials mainly include organic phase change materials, inorganic hydrated salt materials, and eutectic phase change materials. Although organic phase change materials have good chemical stability, they generally suffer from low thermal conductivity, limited latent heat of phase change, and limited temperature range, making it difficult to balance thermal storage density and response speed in electrothermal conversion applications. Eutectic phase change materials, through multi-component synergy, can achieve precise control of phase change temperature, showing great application potential in the mid-temperature thermal storage field (Islam A, Pandey AK, Saidur R, et al. Shape stable composite phase change material with improved thermal conductivity for electrical-to-thermal energy conversion and storage[J]. Materials Today Sustainability, 2024, 25:100678.).

[0005] The eutectic system formed by oxalic acid dihydrate and boric acid has a suitable phase change temperature and high latent heat in the mid-temperature range, which can cover part of the typical operating temperature range of electrothermal conversion systems, and has the basic conditions to be used as a mid-temperature phase change thermal storage material. However, this type of eutectic phase change material is prone to molten state migration and liquid leakage during the phase change process. Especially under electrothermal conversion conditions, the material often undergoes a rapid heating process and the molten state lasts for a long time, which further aggravates the leakage risk, leading to problems such as unstable material morphology, latent heat decay and structural damage, which seriously restricts its engineering application (Xie S, Sun J, Wang Z, et al. A thermally stable phase change material with highlatent heat based on an oxalic acid dihydrate / boric acid binary eutectic system[J]. Solar Energy Materials and Solar Cells, 2017, 168: 38-44.).

[0006] To address these issues, existing technologies often employ porous or thermally conductive materials to modify phase change materials. For example, introducing expanded graphite, foamed metal, or porous inorganic carriers can improve the material's thermal conductivity and thermal response during electrothermal conversion to some extent. However, relying solely on thermally conductive materials is insufficient to effectively suppress the liquid migration of molten phase change materials, especially under conditions of repeated electrothermal cycles or rapid heating, where significant leakage risks still exist.

[0007] Fumed silica, as a nanoscale porous material, possesses characteristics such as small particle size, large specific surface area, and abundant hydroxyl groups on its surface. It can effectively confine molten phase change materials through adsorption in nanopores and interfacial interactions. Expanded graphite, with its layered structure and excellent thermal conductivity, can construct continuous thermal pathways within the composite material, improving the thermal response speed and temperature uniformity during electrothermal conversion. However, current technologies lack a systematic solution for the synergistic application of the nano-confining stabilizing effect of fumed silica and the thermal conductivity-enhancing effect of expanded graphite in the oxalic acid dihydrate-boric acid eutectic system.

[0008] Therefore, there is an urgent need to provide a composite phase change thermal storage material suitable for the medium temperature range, especially capable of meeting the operating temperature range requirements of electrothermal conversion systems. This material should maintain a high thermal storage density while also possessing good thermal conductivity and morphological stability to meet the comprehensive requirements of safety, reliability, and cycle stability in practical engineering applications. Summary of the Invention

[0009] The main objective of this invention is to provide a dihydrate oxalic acid-boric acid eutectic phase change composite material, its preparation method, and its application. This method can achieve the control of phase change temperature and phase change enthalpy and the improvement of thermal stability of the eutectic phase change material.

[0010] To achieve the above-mentioned objectives, the present invention provides the following technical solutions.

[0011] This invention provides a dihydrate oxalic acid-boric acid eutectic phase change composite material, comprising: a eutectic phase change material component, a porous adsorption support, and a conductive filler; wherein the eutectic phase change material component is boric acid and dihydrate oxalic acid, the porous adsorption support is fumed silica, and the conductive filler is expanded graphite.

[0012] Preferably, the eutectic phase change material composition, by mass parts, includes 88 parts of oxalic acid dihydrate and 12 parts of boric acid.

[0013] Further preferred, the purity of boric acid is above 99%, and the purity of oxalic acid dihydrate is above 99%.

[0014] Preferably, the dioxacid-boric acid dihydrate eutectic phase change composite material includes dioxacid-boric acid dihydrate / silica eutectic phase change material. More preferably, the mass ratio of porous adsorbent carrier: conductive filler: eutectic phase change material component in oxalic acid dihydrate-boric acid / silica eutectic phase change material is 5-35:0:65-95; the sum of the mass parts of porous adsorbent carrier, conductive filler and eutectic phase change material component is 100 parts.

[0015] In a further preferred embodiment, the oxalic acid dihydrate-boric acid / silica eutectic phase change material has a mass ratio of porous adsorbent carrier: conductive filler: eutectic phase change material component of 10-25:0:75-90; and the sum of the mass parts of porous adsorbent carrier, conductive filler and eutectic phase change material component is 100 parts.

[0016] The most preferred embodiment is that the mass ratio of porous adsorbent carrier: conductive filler: eutectic phase change material component in oxalic acid dihydrate-boric acid / silica eutectic phase change material is 15:0:85; and the sum of the mass parts of porous adsorbent carrier, conductive filler and eutectic phase change material component is 100 parts.

[0017] Preferably, the dioxacid-boric acid dihydrate eutectic phase change composite material includes dioxacid-boric acid dihydrate / silica / expanded graphite eutectic phase change material.

[0018] In a further preferred embodiment, the oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material has a mass ratio of porous adsorbent carrier: conductive filler: eutectic phase change material component of 6~14:1~9:85; and the sum of the mass parts of porous adsorbent carrier, conductive filler and eutectic phase change material component is 100 parts.

[0019] In a further preferred embodiment, the oxalic acid-boric acid dihydrate / silica / expanded graphite eutectic phase change material has a mass ratio of porous adsorbent carrier: conductive filler: eutectic phase change material component of 6:9:85; and the sum of the mass parts of porous adsorbent carrier, conductive filler and eutectic phase change material component is 100 parts.

[0020] This invention also provides a method for preparing the above-mentioned oxalic acid dihydrate-boric acid eutectic phase change composite material, comprising the following steps: (1) Boric acid and oxalic acid dihydrate are mixed and heated to melt, resulting in a clear and transparent molten mixture, which is the component of the eutectic phase change material; (2) Add the porous adsorption carrier and conductive filler to the molten mixture and continue stirring; (3) Cool the molten mixture obtained in step (2) to crystallize and obtain oxalic acid dihydrate-boric acid eutectic phase change composite material.

[0021] Preferably, the heating and melting temperature in step (1) is 105°C.

[0022] Preferably, the cooling condition in step (3) is sealed and the cooling temperature is room temperature.

[0023] The present invention also provides the application of the above-mentioned dihydrate oxalic acid-boric acid eutectic phase change composite material in medium-temperature heat storage and electrothermal conversion applications.

[0024] Compared with the prior art, the present invention has the following significant advantages: (1) This invention adds fumed silica and expanded graphite to a composite of oxalic acid-boric acid dihydrate eutectic phase change material, adjusting the phase change temperature to approximately 86°C, and solving the problems of poor thermal stability, poor shape stability, and poor cycle stability, while ensuring the retention of latent heat. This lays the foundation for the research and improvement of the thermal properties of eutectic phase change materials.

[0025] (2) The phase transition temperature and phase transition enthalpy of the dihydrate oxalic acid-boric acid / silica / expanded graphite eutectic phase change material prepared by the present invention can be controlled and can be applied to medium-temperature heat storage and electrothermal conversion applications.

[0026] (3) The phase change enthalpy of the dihydrate oxalic acid-boric acid eutectic phase change composite material prepared by the present invention is still at a high level, with no phase separation, strong shape stability, reliable thermal performance, and strong thermal cycling stability.

[0027] (4) The preparation method provided by the present invention is simple, the raw materials are inexpensive, and the preparation conditions are mild, making it suitable for large-scale industrial production.

[0028] In summary, the oxalic acid-boric acid dihydrate eutectic phase change composite material based on fumed silica and expanded graphite reinforced by this invention solves the problems of severe liquid leakage and insufficient stability in traditional systems, and provides an innovative high-performance solution for the development and application of medium-temperature thermal storage materials. Attached Figure Description

[0029] Figure 1 The DSC thermal performance analysis curves are shown for the oxalic acid-boric acid dihydrate eutectic phase change material prepared in Example 1, the oxalic acid-boric acid dihydrate / silica eutectic phase change materials prepared in Examples 2-5, and the oxalic acid-boric acid dihydrate / silica / expanded graphite eutectic phase change material prepared in Example 10.

[0030] Figure 2 Comparison of liquid leakage test results for the dihydrate oxalic acid-boric acid / silica / expanded graphite eutectic phase change materials prepared in Examples 6-10.

[0031] Figure 3 Infrared spectra of fumed silica, expanded graphite, oxalic acid-boric acid dihydrate eutectic phase change material prepared in Example 1, oxalic acid-boric acid dihydrate / silica eutectic phase change material prepared in Example 4, and oxalic acid-boric acid dihydrate / silica / expanded graphite eutectic phase change material prepared in Example 10.

[0032] Figure 4 The thermal conductivity diagrams are for the oxalic acid-boric acid dihydrate / silica / expanded graphite eutectic phase change materials prepared in Examples 6-10.

[0033] Figure 5The resistivity diagrams are for the oxalic acid-boric acid dihydrate / silica / expanded graphite eutectic phase change materials prepared in Examples 6-10.

[0034] Figure 6 The temperature rise curves of the dihydrate oxalic acid-boric acid / silica / expanded graphite eutectic phase change materials prepared in Examples 8-10 under a constant voltage charging condition of 4V. Detailed Implementation

[0035] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention... Examples, and all other embodiments obtained by those skilled in the art without inventive effort, are considered... Within the scope of protection of this invention.

[0036] Example 1 Based on mass parts, the oxalic acid dihydrate-boric acid eutectic phase change material of this Example 1 was made from the following raw materials: 88 parts of oxalic acid dihydrate and 12 parts of boric acid; The preparation method of oxalic acid dihydrate-boric acid eutectic phase change material includes the following steps: (1) Oxalic acid dihydrate and boric acid are mixed evenly at a mass ratio of 88:12 and placed in a constant temperature water bath at 105℃. The mixture is heated at 105℃ to melt it. (2) Stir magnetically for 30 minutes to obtain a clear and transparent eutectic; (3) Under sealed conditions, the liquid oxalic acid dihydrate-boric acid eutectic phase change material is cooled and crystallized at room temperature to obtain a white eutectic, which is the oxalic acid dihydrate-boric acid eutectic phase change material, and is called oxalic acid dihydrate. Boric acid eutectic phase change material was ground into powder and subjected to DSC testing.

[0037] Finally, the oxalic acid dihydrate was measured. The melting temperature of the boric acid eutectic phase change material is 87.78℃, and the enthalpy of melting is 341J / g. Example 2 The preparation method of oxalic acid dihydrate-boric acid / silica eutectic phase change material includes the following steps: (1) Oxalic acid dihydrate and boric acid are mixed evenly at a mass ratio of 88:12 and placed in a constant temperature water bath at 105℃. The mixture is heated at 105℃ to melt it. (2) Stir with magnetic force for 30 minutes to obtain a clear and transparent eutectic melt, which is the composition of the eutectic phase change material; (3) Add fumed silica to the eutectic phase change material at a mass ratio of 5:95 and continue stirring; (4) Under sealed conditions, the liquid mixture obtained in step (3) is cooled and crystallized at room temperature to obtain a white eutectic, which is the oxalic acid dihydrate-boric acid / silica eutectic phase change material, called 0.05 silica. It is ground into powder for DSC testing.

[0038] The final measured melting temperature of 0.05% silicon dioxide was 87.14℃, and the enthalpy of melting was 327.4 J / g.

[0039] Example 3 The preparation method of oxalic acid dihydrate-boric acid / silica eutectic phase change material includes the following steps: (1) Oxalic acid dihydrate and boric acid are mixed evenly at a mass ratio of 88:12 and placed in a constant temperature water bath at 105℃. The mixture is heated at 105℃ to melt it. (2) Stir magnetically for 30 minutes to obtain a clear and transparent eutectic; (3) Add fumed silica to the eutectic phase change material at a mass ratio of 10:90 and continue stirring; (4) Under sealed conditions, the liquid mixture obtained in step (3) is cooled and crystallized at room temperature to obtain a white eutectic, which is the oxalic acid-boric acid / silica eutectic phase change material, called 0.1 silica. It is ground into powder and subjected to DSC test.

[0040] The final measured melting temperature of 0.1% silicon dioxide was 86.74℃, and the enthalpy of melting was 304 J / g.

[0041] Example 4 The preparation method of oxalic acid dihydrate-boric acid / silica eutectic phase change material includes the following steps: (1) Oxalic acid dihydrate and boric acid are mixed evenly at a mass ratio of 88:12 and placed in a constant temperature water bath at 105℃. The mixture is heated at 105℃ to melt it. (2) Stir magnetically for 30 minutes to obtain a clear and transparent eutectic; (3) Add fumed silica to the eutectic phase change material at a mass ratio of 15:85 and continue stirring; (4) Under sealed conditions, the liquid mixture obtained in step (3) is cooled and crystallized at room temperature to obtain a white eutectic, which is the oxalic acid-boric acid / silica eutectic phase change material, called 0.15 silica. It is ground into powder and subjected to DSC test.

[0042] The final measured melting temperature of 0.15% silicon dioxide was 85.79℃, and the enthalpy of melting was 290.4 J / g.

[0043] Example 5 The preparation method of oxalic acid dihydrate-boric acid / silica eutectic phase change material includes the following steps: (1) Oxalic acid dihydrate and boric acid are mixed evenly at a mass ratio of 88:12 and placed in a constant temperature water bath at 105℃. The mixture is heated at 105℃ to melt it. (2) Stir magnetically for 30 minutes to obtain a clear and transparent eutectic; (3) Add fumed silica to the eutectic phase change material at a mass ratio of 20:80 and continue stirring; (4) Under sealed conditions, the liquid mixture obtained in step (3) is cooled and crystallized at room temperature to obtain a white eutectic, which is the oxalic acid dihydrate-boric acid / silica eutectic phase change material, called 0.02 silica. It is ground into powder and subjected to DSC test.

[0044] The final measured melting temperature of 0.2% silicon dioxide was 84.17℃, and the enthalpy of melting was 265.4 J / g.

[0045] Figure 1 The DSC thermal performance analysis curves of the oxalic acid-boric acid dihydrate eutectic phase change material prepared in Example 1, the oxalic acid-boric acid dihydrate / silica eutectic phase change materials prepared in Examples 2-5, and the oxalic acid-boric acid dihydrate / silica / expanded graphite eutectic phase change material prepared in Example 10 are shown. 0.15% silica with higher enthalpy and stability was selected as the proportion for subsequent studies.

[0046] Example 6 The preparation method of oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material includes the following steps: (1) Oxalic acid dihydrate and boric acid are mixed evenly at a mass ratio of 88:12 and placed in a constant temperature water bath at 105℃. The mixture is heated at 105℃ to melt it. (2) Stir magnetically for 30 minutes to obtain a clear and transparent eutectic; (3) Stir the expanded graphite and fumed silica with the eutectic phase change material in a mass ratio of 14:1:85. (4) Under sealed conditions, the mixture obtained in step (3) is cooled and crystallized at room temperature to obtain oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material, which is called 14% silica-1% expanded graphite.

[0047] Example 7 The preparation method of oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material includes the following steps: (1) Oxalic acid dihydrate and boric acid are mixed evenly at a mass ratio of 88:12 and placed in a constant temperature water bath at 105℃. The mixture is heated at 105℃ to melt it. (2) Stir magnetically for 30 minutes to obtain a clear and transparent eutectic; (3) Stir the expanded graphite and fumed silica with the eutectic phase change material in a mass ratio of 12:3:85. (4) Under sealed conditions, the mixture obtained in step (3) is cooled and crystallized at room temperature to obtain oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material, which is called 12% silica-3% expanded graphite.

[0048] Example 8 The preparation method of oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material includes the following steps: (1) Oxalic acid dihydrate and boric acid are mixed evenly at a mass ratio of 88:12 and placed in a constant temperature water bath at 105℃. The mixture is heated at 105℃ to melt it. (2) Stir magnetically for 30 minutes to obtain a clear and transparent eutectic; (3) Stir the expanded graphite and fumed silica with the eutectic phase change material in a mass ratio of 10:5:85. (4) Under sealed conditions, the mixture obtained in step (3) is cooled and crystallized at room temperature to obtain oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material, which is called 10% silica-5% expanded graphite.

[0049] Example 9 The preparation method of oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material includes the following steps: (1) Oxalic acid dihydrate and boric acid are mixed evenly at a mass ratio of 88:12 and placed in a constant temperature water bath at 105℃. The mixture is heated at 105℃ to melt it. (2) Stir magnetically for 30 minutes to obtain a clear and transparent eutectic; (3) Stir the expanded graphite and fumed silica with the eutectic phase change material in a mass ratio of 8:7:85. (4) Under sealed conditions, the mixture obtained in step (3) is cooled and crystallized at room temperature to obtain oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material, which is called 8% silica-7% expanded graphite.

[0050] Example 10 The preparation method of oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material includes the following steps: (1) Oxalic acid dihydrate and boric acid are mixed evenly at a mass ratio of 88:12 and placed in a constant temperature water bath at 105℃. The mixture is heated at 105℃ to melt it. (2) Stir magnetically for 30 minutes to obtain a clear and transparent eutectic; (3) Stir the expanded graphite and the eutectic phase change material in a mass ratio of 6:9:85. (4) Under sealed conditions, the mixture obtained in step (3) is cooled and crystallized at room temperature to obtain oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material, which is called 6% silica-9% expanded graphite.

[0051] Figure 2 The results of liquid leakage tests on the oxalic acid-boric acid dihydrate / silica / expanded graphite eutectic phase change materials prepared in Examples 6-10 are shown in comparison. Figure 2 It was found that the shape and morphology of all oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material bulks remained almost unchanged after the liquid leakage experiment, indicating stable bulk shape. The mass change rate of the samples was less than 5%, and the change was minimal. The volume change rate was also less than 5%, indicating that the material still has good adsorption capacity after the addition of EG, and can prevent liquid leakage of the phase change material.

[0052] Figure 3 Infrared spectra of fumed silica, expanded graphite, the oxalic acid-boric acid dihydrate eutectic phase change material prepared in Example 1, the oxalic acid-boric acid / silica eutectic phase change material prepared in Example 4, and the oxalic acid-boric acid / silica / expanded graphite eutectic phase change material prepared in Example 10 are shown. Figure 3 It can be seen that the infrared spectrum of the oxalic acid dihydrate-boric acid / silica / expanded graphite eutectic phase change material contains all the characteristic peaks of oxalic acid dihydrate, boric acid, silica, and expanded graphite, respectively, and no new characteristic peaks appear. This indicates that the adsorption of oxalic acid dihydrate-boric acid eutectic material by silica and its composite with expanded graphite are physical processes, and no new functional groups are generated.

[0053] Figure 4 The thermal conductivity of the oxalic acid-boric acid dihydrate / silica / expanded graphite eutectic phase change materials prepared in Examples 6-10 is shown. Figure 4 It can be seen that the thermal conductivity increases significantly with the increase of expanded graphite content, reaching a maximum of 4.438 W / m·K. This creates thermal conductivity pathways within the bulk material.

[0054] Figure 5 The resistivity of the oxalic acid-boric acid dihydrate / silica / expanded graphite eutectic phase change materials prepared in Examples 6-10 is shown. Figure 5It can be seen that the volume resistivity of the bulk material is negatively correlated with the amount of EG used. When the EG content increases from 1 wt% to 3 wt%, the volume resistivity drops sharply to 9.774 Ω·cm. When the content exceeds 5 wt%, the rate of change of resistivity decreases with increasing EG content.

[0055] Figure 6 The temperature rise curves of the oxalic acid-boric acid dihydrate / silica / expanded graphite eutectic phase change materials prepared in Examples 8-10 are shown under a 4V constant voltage charging condition. Figure 6 It can be seen that as the heating process proceeds, the electrothermal conversion module undergoes sensible heat storage and latent heat storage stages. The heating rate shows a trend of first decreasing and then increasing, with a small plateau in between. This is because it enters the phase transition stage, and the apparent specific heat capacity of the material increases sharply. The heat obtained from the electrothermal conversion is stored in the material in the form of latent heat storage. The 6% silica-9% expanded graphite sample has the fastest temperature rise rate. The faster the phase transition is completed, the lower the resistance under the same voltage, and the faster the phase transition is completed.

[0056] This invention successfully constructed a eutectic composite phase change material with silica as the adsorption framework and expanded graphite as the conductive filler. The material maintains the high phase transition enthalpy of the eutectic system through physical composite composition. Liquid leakage testing shows excellent shape stability, with both mass and volume change rates below 5%, demonstrating good liquid leakage suppression capabilities. The material's thermal conductivity is significantly improved (reaching a maximum of 4.438 W / m·K), and a conductive network is formed, resulting in a substantial reduction in volume resistivity. Constant voltage charging testing confirms its efficient electrothermal conversion and storage capabilities; samples with lower resistance complete the phase transition faster. In summary, this material integrates high heat storage density, excellent shape stability, rapid thermal response, and efficient electrothermal conversion, showing broad application prospects in the field of thermal energy management.

[0057] The above embodiments are used to explain and illustrate the present invention. The implementation of the present invention is not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the claims of the present invention.

Claims

1. A dihydrate oxalic acid-boric acid eutectic phase change composite material, characterized in that, include: eutectic phase change material components, porous adsorption carriers, and conductive fillers; The eutectic phase change material consists of boric acid and oxalic acid dihydrate, the porous adsorption carrier is fumed silica, and the conductive filler is expanded graphite.

2. The oxalic acid dihydrate-boric acid eutectic phase change composite material according to claim 1, characterized in that, The eutectic phase change material composition, by mass parts, includes 88 parts of oxalic acid dihydrate and 12 parts of boric acid.

3. The oxalic acid dihydrate-boric acid eutectic phase change composite material according to claim 2, characterized in that, The purity of the boric acid is above 99%, and the purity of the oxalic acid dihydrate is above 99%.

4. The oxalic acid dihydrate-boric acid eutectic phase change composite material according to claim 1, characterized in that, The dioxacid-boric acid dihydrate eutectic phase change composite material includes dioxacid-boric acid dihydrate / silica eutectic phase change material.

5. The oxalic acid dihydrate-boric acid eutectic phase change composite material according to claim 4, characterized in that, The mass ratio of porous adsorption carrier: conductive filler: eutectic phase change material component in the dihydrate oxalic acid-boric acid / silica eutectic phase change material is 5-35:0:65-95; the sum of the mass parts of porous adsorption carrier, conductive filler and eutectic phase change material component is 100 parts.

6. The oxalic acid dihydrate-boric acid eutectic phase change composite material according to claim 1, characterized in that, The dioxacid-boric acid dihydrate eutectic phase change composite material includes dioxacid-boric acid dihydrate / silica / expanded graphite eutectic phase change material.

7. The oxalic acid dihydrate-boric acid eutectic phase change composite material according to claim 6, characterized in that, The mass ratio of porous adsorption carrier: conductive filler: eutectic phase change material in the dihydrate oxalic acid-boric acid / silica / expanded graphite eutectic phase change material is 6~14:1~9:85; the sum of the mass parts of the porous adsorption carrier, conductive filler and eutectic phase change material is 100 parts.

8. Claim 1 The method for preparing the dihydrate oxalic acid-boric acid eutectic phase change composite material according to any one of the seven claims is characterized in that, Includes the following steps: (1) Boric acid and oxalic acid dihydrate are mixed and heated to melt, resulting in a clear and transparent molten mixture, which is the component of the eutectic phase change material; (2) Add the porous adsorption carrier and conductive filler to the molten mixture and continue stirring; (3) Cool the molten mixture obtained in step (2) to crystallize and obtain oxalic acid dihydrate-boric acid eutectic phase change composite material.

9. The preparation method according to claim 8, characterized in that, The cooling conditions described in step (3) are sealed and the cooling temperature is room temperature.

10. The application of the dihydrate oxalic acid-boric acid eutectic phase change composite material according to any one of claims 1 to 7 in medium-temperature heat storage and electrothermal conversion applications.

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