Anti-circulation fatigue packaging structure plastic crystal pressure card material and preparation method

Abstract

This invention relates to the field of solid-state refrigeration materials with pressure-sensitive effects, and discloses a technical method for improving the adiabatic temperature change decay of plastic crystal materials under constant pressure cycling conditions. The method involves uniformly mixing ammonium iodide plastic crystal grains with silicone oil to form a coating composite system with lubricating and isolating functions. A spherical shell encapsulation structure is then fabricated using PA12 nylon powder via laser sintering 3D printing. The composite system is filled and sealed within the spherical shell to form a stable encapsulation unit. During programmed pressurization and depressurization cycles of the encapsulation unit under constant pressure conditions, the silicone oil effectively reduces inter-grain friction and stress concentration, while the spherical shell structure prevents the loss of the coating medium and maintains the integrity of the system, thereby significantly suppressing the decay of the adiabatic temperature change of the plastic crystal material with increasing cycle count.

Description

Technical Field

[0001] This invention belongs to the field of solid-state refrigeration and pressure-clamping material technology, specifically relating to a method and apparatus for improving the fatigue resistance of plastic crystal pressure-clamping materials during constant pressure cyclic loading, which reduces adiabatic temperature change attenuation. Background Technology

[0002] The increasingly severe global energy shortage and climate change have become significant factors restricting sustainable socio-economic development. Refrigeration and air conditioning systems, as a crucial component of end-use energy consumption, have garnered widespread attention for their energy efficiency and environmental impact. While existing mainstream vapor compression refrigeration technology is relatively mature in engineering applications, it relies heavily on refrigerants containing fluorinated compounds, which have high global warming potential. Leaks during use and maintenance can easily cause adverse effects on the atmospheric environment and climate system, contradicting current low-carbon and green development goals. Therefore, developing efficient and environmentally friendly new refrigeration technologies to reduce reliance on high-greenhouse-effect refrigerants has become an urgent technical challenge in the refrigeration field.

[0003] Solid-state phase change-based refrigeration technology, due to its absence of gas compression and volatilization processes, offers advantages such as zero greenhouse gas emissions, relatively simple system structure, high operational reliability, and potential high energy efficiency, making it a significant development direction for replacing traditional refrigeration technologies. Among these technologies, the barocalorice effect, which induces a first-order phase change in materials using applied pressure, has received widespread attention in recent years. This effect causes a phase transition within the material through pressure changes, resulting in significant entropy and temperature changes, thereby achieving heat absorption or release. Compared to other solid-state refrigeration methods such as magnetic cards and electronic cards, barocalorice offers a wider variety of materials, more flexible response to external fields, and greater design freedom and engineering application potential.

[0004] Among existing pressure-clamping materials, plastic crystals, due to their somewhat disordered molecular or ionic orientations, relatively weak interactions, and ease of structural rearrangement, exhibit significant pressure-clamping effects, resulting in substantial configurational entropy changes under external pressure. This has made them an important research subject in the field of solid-state refrigeration in recent years. Ammonium iodide is a typical plastic crystal pressure-clamping material, undergoing a pressure-induced phase transition near room temperature, accompanied by large isothermal entropy changes and adiabatic temperature changes. Due to the highly disordered orientation of ammonium ions, this material exhibits a high entropy change amplitude during the phase transition, while its phase transition driving pressure is relatively low, demonstrating potential for engineering applications.

[0005] However, existing research and practical applications show that under repeated pressurization and depressurization cycles, the adiabatic temperature change of plastic crystal pressing materials such as ammonium iodide significantly decreases with increasing cycle count, exhibiting insufficient cyclic stability. This phenomenon is usually related to factors such as increased interparticle friction, continuous accumulation of local structural defects, and increased energy dissipation during phase transitions, leading to some irreversible or limited phase transitions, thereby weakening the material's effective pressing effect. These problems severely restrict the performance maintenance of plastic crystal pressing materials under long-term stable operating conditions, becoming a key technical bottleneck that urgently needs to be addressed for their engineering applications.

[0006] To address the aforementioned shortcomings, current technologies lack a structural design scheme that can effectively alleviate interparticle interactions, reduce stress concentration during pressure cycling, and improve cycle stability in plastic crystal accelerator materials without significantly weakening their intrinsic accelering properties. Therefore, a new technical approach is urgently needed to improve the problem of adiabatic temperature change in plastic crystal accelerator materials decreasing with increasing cycle count during repeated pressurization and depressurization, providing reliable technical support for their practical application in solid-state refrigeration.

[0007] To address this, the present invention proposes an improved solution. By introducing a flowable coating medium into the plastic crystal pressing material system and combining it with an encapsulation structure to physically constrain the entire material, effective isolation between particles is achieved. This reduces friction and localized stress concentration during cyclic loading, significantly improving the cyclic decay behavior of adiabatic temperature changes. While maintaining the inherent advantages of plastic crystal materials, such as high entropy change amplitude and low driving pressure, this solution effectively enhances their cyclic stability, providing a new approach for the engineering application of solid-state phase change refrigeration technology. Summary of the Invention

[0008] The purpose of this invention is to provide a technical method for improving the adiabatic temperature change decay of plastic crystal materials under constant pressure cycling due to the compression effect. Specifically, it addresses the problem of thermal effect decay of ammonium iodide plastic crystal materials during repeated pressurization and depressurization processes caused by interparticle friction and stress concentration. An improved scheme with both structural constraint and lubrication functions is proposed. This method disperses plastic crystal particles in silicone oil to form a coating system, and further encapsulates the entire system using 3D-printed PA12 spherical shells. This effectively suppresses the extrusion and loss of silicone oil during cyclic pressurization, reduces friction and defect accumulation caused by direct contact between particles, and achieves a significant improvement in adiabatic temperature change decay.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A method for improving the thermal degradation of ammonium iodide plastic crystal materials includes the following steps: First, ammonium iodide particles are uniformly mixed with silicone oil to fully disperse the ammonium iodide particles in the silicone oil, forming a stable silicone oil-coated composite system; Second, a hollow spherical shell structure matching the sample is prepared using PA12 nylon powder through 3D printing, and the silicone oil-coated composite system is filled into the hollow spherical shell and sealed to obtain a coated unit; Finally, the coated unit is subjected to multiple pressurization-depressurization cycles under program control, with the peak pressure of the pressurization-depressurization cycle being 80-100 MPa. Each cycle includes: pressurizing to the peak pressure within 3-5 seconds, then exchanging heat with the environment for a fixed duration of 30 seconds as set by the instrument program, then depressurizing within 3-5 seconds, and continuing heat exchange for another 30 seconds after depressurization. The silicone oil plays a lubricating and isolating role during the pressurization-depressurization process, which can reduce friction and stress accumulation between particles, thereby reducing the attenuation of thermal insulation temperature change and improving cycle stability.

[0010] During programmed pressurization and depressurization cycles of the encapsulation unit under constant pressure, silicone oil can effectively reduce intergranular friction and stress concentration, and the spherical shell structure can prevent the loss of the coating medium and maintain the integrity of the system, thereby significantly suppressing the attenuation of the thermal insulation temperature change of the plastic crystal material with the increase of the number of cycles.

[0011] Through the above-described scheme, this invention utilizes the lubricating and isolating effects of silicone oil on the particle surface to significantly reduce the friction coefficient and stress concentration effect between ammonium iodide particles, slowing down the accumulation of structural defects and thus improving the thermodynamic stability of the material during multiple pressurization cycles. Simultaneously, the PA12 nylon 3D-printed spherical shell provides excellent mechanical support and encapsulation constraints, preventing silicone oil extrusion and leakage during pressurization, and maintaining the integrity and uniformity of the coating system. Tests show that the method described in this invention retains more than 60% of the initial adiabatic temperature change after 50 pressurization cycles and more than 50% after 100 cycles, significantly outperforming the performance of uncoated samples and demonstrating good cycle durability and application prospects. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the overall process for preparing the plastic crystal press-fit material, which is the packaging structure of the improved method for thermal insulation temperature change attenuation of plastic crystal press-fit material according to the present invention. Figure 2 The temperature versus time curve of the coating unit with an NH4I to silicone oil mass ratio of 9:1 in the example during the cyclic pressurization-depressurization process; Figure 3 The temperature versus time curve of the pure phase NH4I sample in Comparative Example 1 during the cyclic pressurization-depressurization process; Figure 4The temperature versus time curves of the coating unit with an NH4I to silicone oil mass ratio of 9.5:0.5 in Comparative Example 2 during the cyclic pressurization-depressurization process are shown. Figure 5 The temperature versus time curves of the coating unit with an NH4I to silicone oil mass ratio of 8:2 in Comparative Example 3 during the cyclic pressurization-depressurization process are shown. Figure 6 The adiabatic temperature change curves of Example 1 and each comparative sample during the cyclic pressurization-depressurization process as a function of the number of cycles. Detailed Implementation

[0013] To further understand the present invention, the following description, in conjunction with the accompanying drawings and specific embodiments, will provide further details.

[0014] Example 1

[0015] Ammonium iodide particles were used as the raw material for plastic crystal formation, with the particle size controlled within the range of 400–500 mm. AP100 type silicone oil was used as the coating medium. The raw materials were weighed at a mass ratio of ammonium iodide to silicone oil of 9:1 and placed in a mixing container for mechanical stirring for 15 min to ensure that the ammonium iodide particles were fully and uniformly dispersed in the silicone oil, forming a stable silicone oil-coated ammonium iodide composite system. The AP100 type silicone oil is a polydimethylsiloxane-based silicone oil with a viscosity of 140 mPa·s at 25 °C. After mixing, the resulting composite system was placed in a fixed mold and pressed under 10 MPa pressure for 3 min to form spherical blocks with a diameter of 6 mm.

[0016] Subsequently, using PA12 nylon powder as raw material, a hollow spherical shell structure matching the size of the spherical block was prepared by selective laser sintering 3D printing. The PA12 spherical shell has an inner diameter of 6.2 mm and a wall thickness of 0.8 mm, and consists of upper and lower hemispherical shells. The opening ends of the upper and lower hemispherical shells are connected by a stop fit, that is, the edge of the opening end of the upper hemisphere is provided with a raised step, and the edge of the opening end of the lower hemisphere is provided with a groove corresponding to the raised step. During assembly, the two are aligned vertically and fixed by elastic interference fit and friction. The spherical block is then inserted into the PA12 spherical shell to obtain a complete covering unit, the structure of which is as follows. Figure 1 As shown.

[0017] Finally, the coating unit underwent a cyclic pressurization-depressurization test. Specifically, the coating unit was subjected to multiple pressurization-depressurization cycles controlled by a program, with a peak cyclic pressure of 100 MPa. Each cycle included: pressurizing to 100 MPa within 4 seconds, holding for 30 seconds for heat exchange with the environment, depressurizing to atmospheric pressure within 4 seconds, and holding for another 30 seconds for heat exchange after depressurization. The temperature-time relationship curve of the coating unit during the cyclic pressurization-depressurization process is shown below. Figure 2 As shown.

[0018] Comparative Example 1 To investigate the cycling stability of the samples without silicone oil coating or encapsulation, a pure-phase NH4I sample was prepared as Comparative Example 1. Ammonium iodide particles were used as the molding raw material, with the particle size controlled to be 400–500 mm. The ammonium iodide particles were placed directly in a fixed mold and pressed at 10 MPa for 3 min to form spherical sample units with a diameter of 6 mm. No silicone oil was added in Comparative Example 1, and no PA12 spherical shell encapsulation structure was used.

[0019] Finally, a cyclic pressurization-depressurization test was conducted under the same conditions as in Example 1, and the temperature change of the sample during the cycle was recorded. The temperature versus time curve is shown below. Figure 3 As shown.

[0020] Comparative Example 2 The process and conditions are the same as in Example 1, except that: To investigate the effect of low silicone oil content on the cycling performance of the samples, a coating unit with an ammonium iodide to silicone oil mass ratio of 9.5:0.5 was prepared as Comparative Example 2. Ammonium iodide particles and AP100 silicone oil were weighed at a mass ratio of 9.5:0.5 and placed in a mixing container. The mixture was stirred for 15–20 min under the same mechanical stirring conditions as in Example 1 to form a silicone oil-coated ammonium iodide composite system. Subsequently, under the same pressing conditions as in Example 1, the resulting composite system was pressed into spherical blocks with a diameter of 6 mm, and then inserted into hollow PA12 spherical shells of the same structure and size as in Example 1 to form the coating unit. The test was then conducted under the same cyclic pressurization-depressurization conditions as in Example 1. The temperature versus time curve is shown below. Figure 4 As shown.

[0021] Comparative Example 3 The process and conditions are the same as in Example 1, except that: To investigate the effect of high silicone oil content on the cycling performance of the samples, a coating unit with an ammonium iodide to silicone oil mass ratio of 8:2 was prepared as Comparative Example 3. Ammonium iodide particles and AP100 silicone oil were weighed at a mass ratio of 8:2 and placed in a mixing container. The mixture was stirred for 15–20 min under the same mechanical stirring conditions as in Example 1 to form a silicone oil-coated ammonium iodide composite system. Subsequently, under the same pressing conditions as in Example 1, the resulting composite system was pressed into spherical blocks with a diameter of 6 mm and inserted into hollow PA12 spherical shells of the same structure and size as in Example 1 to form the coating unit. The test was then conducted under the same cyclic pressurization-depressurization conditions as in Example 1. The temperature versus time curve is shown below. Figure 5 As shown.

[0022] Samples from Example 1 and Comparative Examples 1-3 were placed in an adiabatic temperature change testing device of a hydraulic press system for cyclic loading and unloading tests. Details of the adiabatic temperature change testing device can be found in the doctoral dissertation (Yu Chenyang. Phase Transition and Thermophysical Properties of Formamide and Methylamine Halides [D]. University of Science and Technology of China, 2024. DOI:10.27517 / d.cnki.gzkju.2024.000142.). Before testing, thermocouples were placed at the core of the sample to collect real-time temperature change signals; simultaneously, the sample was kept in thermal equilibrium under initial pressure, and its initial temperature T0 was recorded. During the test, the hydraulic press was controlled by a program to cyclically load and unload the sample under a pressure limit of 100 MPa. During the pressurization phase, pressure loading was completed within 5 seconds, and the sample temperature change was recorded simultaneously. The sample underwent a phase change and released heat during pressurization, resulting in a temperature increase. The peak temperature T1 reached after pressurization, and the difference between T1 and the initial temperature T0, were defined as the adiabatic temperature change (ΔT) during this pressurization process. T a‍d ), that is, Δ T a‍d =T1−T0.

[0023] After pressurization, the system is held for 30 seconds to allow sufficient heat exchange with the external environment and return to ambient temperature. A rapid depressurization operation is then performed, causing a reverse phase transition and endothermic reaction in the sample. Temperature changes during depressurization are recorded. After depressurization, the system is held for another 30 seconds to complete heat exchange and return to its initial temperature. The pressurization, holding, depressurization, and thermal equilibrium processes constitute a complete pressurization-depressurization cycle. This cycle is repeated multiple times, and the adiabatic temperature change is recorded in each cycle.

[0024] Example 1 and Comparative Examples 1-3 were subjected to 100 cycles of testing under the same conditions, and the results were compared as follows: Figure 6As shown in Example 1, the coated sample with an NH4I:silicone oil mass ratio of 9:1 exhibited an adiabatic temperature change of approximately 15.4 K upon initial pressurization, which remained at approximately 7.3 K after the 50th cycle and approximately 5.8 K after the 100th cycle, demonstrating the best cyclic stability. In contrast, the pure-phase NH4I sample in Comparative Example 1 showed an initial adiabatic temperature change of approximately 14 K, which decreased to approximately 2.8 K after the 50th cycle and only about 1.8 K after the 100th cycle; the sample with an NH4I:silicone oil ratio of 9.5:0.5 in Comparative Example 2 showed approximately 3.2 K after the 100th cycle; and the sample with an NH4I:silicone oil ratio of 8:2 in Comparative Example 3 showed approximately 2.3 K after the 100th cycle. These results indicate that the present invention, using a 9:1 mass ratio of silicone oil for coating combined with PA12 spherical shell encapsulation, can more effectively alleviate interparticle friction and stress concentration, reduce adiabatic temperature change decay, and improve the material's resistance to cyclic fatigue.

[0025] Therefore, the "silicone oil coating + 3D printed spherical shell constraint" method proposed in this invention can effectively slow down the cyclic decay of thermal temperature change while ensuring the compressive strength of ammonium iodide plastic crystal material, and improve the reversibility and long-term stability of the material, which has good application prospects.

Claims

1. A method for preparing a plastic die clamping material for a cyclic fatigue-resistant encapsulation structure, characterized in that: Its preparation method includes the following steps: Step 1: The plastic crystal particles are uniformly mixed and dispersed in silicone oil to form a silicone oil-coated plastic crystal composite system; the mass ratio of ammonium iodide to silicone oil is 7-15:1, preferably 8-12:1, and more preferably 9-10:

1. Step two: Press the composite system into a block and fill it into a hollow spherical shell formed by 3D printing of PA12 powder to form a coating unit, which constitutes the plastic crystal press card material of the encapsulation structure.

2. The preparation method according to claim 1, characterized in that: The plastic crystal particles are ammonium iodide particles, and the particle size of the ammonium iodide particles is 50-500 mm, preferably 200-500 mm, and more preferably 400-500 mm.

3. The preparation method according to claim 1, characterized in that: The silicone oil is a polydimethylsiloxane silicone oil, model AP100, with a viscosity of 100-150 mPa·s at 25°C, preferably 120-150 mPa·s, or more preferably 130-150 mPa·s. The composite material is composed of ammonium iodide particles and silicone oil. The silicone oil coats the surface of the particles and forms a continuous oil phase between the particles to reduce mechanical stress concentration and interfacial friction between the particles and improve the material's cycle stability.

4. The preparation method according to claim 1, characterized in that: The pressure block is a spherical pressure block with a diameter of 4-10 mm (preferably 5-8 mm, more preferably 6-7 mm); the PA12 spherical shell is a hollow spherical structure with a wall thickness of 0.5-1 mm (preferably 0.6-0.8 mm), the inner diameter of the spherical shell is larger than the diameter of the spherical pressure block, and the difference between the inner diameter of the spherical shell and the diameter of the spherical pressure block is 0.15-0.3 mm, preferably 0.18-0.25 mm; the PA12 material spherical shell is elastic and can undergo elastic deformation during the pressurization process to form a compression fit with the spherical pressure block.

5. The preparation method according to claim 1, characterized in that: The silicone oil coating layer is formed by mechanical stirring and dispersing for 15 to 20 minutes.

6. The method according to claim 1, characterized in that: The forming pressure of the press block is 10-20 MPa, and the holding time is 30-60 s.

7. The preparation method according to claim 1, characterized in that, The process also includes step three: performing multiple pressurization-depressurization cycles on the coating unit through program control, wherein the peak pressure of the pressurization-depressurization cycle is 80-100 MPa; wherein each cycle includes: pressurizing to the peak pressure within 3-5 seconds, then exchanging heat with the environment for a fixed duration of 20-40 seconds set by the instrument program, then depressurizing to atmospheric pressure within 3-5 seconds, and continuing heat exchange for 20-40 seconds after depressurization.

8. The preparation method according to claim 7, characterized in that, The silicone oil provides lubrication and isolation during the pressure application and depressurization process to reduce friction and stress accumulation between grains, thereby reducing the attenuation of thermal insulation temperature change and improving cycle stability. The key features are: 100 pressurization cycles, and after 50 cycles, the initial adiabatic temperature change amplitude is maintained at more than 60%, and after 100 cycles, the initial adiabatic temperature change amplitude is maintained at more than 50%.

9. A cyclic fatigue-resistant encapsulation structure plastic crystal press card material prepared by the preparation method according to any one of claims 1-8.

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