Phase change materials (PCMs) are a class of materials that can absorb or release a large amount of energy (i.e., phase change enthalpy) during a phase change. Because PCMs utilize latent heat for energy storage, they have high heat storage density, compact heat storage devices, and their temperature remains essentially constant during the phase change process, making them easy to manage. With the increasing global awareness of energy conservation, this characteristic of PCMs has attracted the attention of researchers, and phase change thermal energy storage technology is increasingly shining in the field of energy storage.
I. Introduction to Material Technology Characteristics
Broadly speaking, thermal energy storage technology includes both thermal energy storage and cold energy storage technologies. Thermal energy storage technology includes sensible thermal energy storage and phase change thermal energy storage. Sensible thermal energy storage utilizes the specific heat capacity of the material itself to store/release thermal energy, while phase change thermal energy storage utilizes the heat absorption/release energy conversion process during the phase change of phase change materials (PCMs) to store/release thermal energy. Phase change thermal energy storage materials have advantages such as high heat storage density and small temperature changes during the charging and releasing of heat, attracting widespread attention from scholars both domestically and internationally. Currently, phase change energy storage materials mainly include organic, molten salt, alloy, and composite types. Their phase change forms are mainly four types: solid-solid, solid-liquid, solid-gas, and liquid-gas.
An ideal solid-liquid phase change material should possess the following properties:
(1) High latent heat of fusion, enabling it to store or release a significant amount of heat during the phase change;
(2) Appropriate phase change temperature to meet requirements;
(3) Good reversibility of the solid-liquid phase change, minimizing overcooling or overheating;
(4) High thermal conductivity between the solid and liquid phases;
(5) Minimal expansion and contraction during the solid-liquid phase change process;
(6) High density and specific heat capacity;
(7) Non-toxic and non-corrosive;
(8) Low cost and easy to manufacture.
Compared to solid-liquid phase change materials, solid-solid phase change materials have many advantages. Solid-solid phase change materials (SCTs) can be directly processed and molded without the need for containers; they have a small coefficient of thermal expansion, resulting in minimal volume change during phase transition; they do not exhibit supercooling or phase separation, eliminating the need for anti-supercooling agents and anti-phase separation agents; they have very low toxicity and minimal corrosivity; they are leak-free and do not pollute the environment; they have stable composition, good phase change reversibility, and a long service life; and their devices are simple and easy to use. The main disadvantages of SCTs are their low latent heat of phase change and high price. Liquid-gas and solid-gas phase change materials, due to the presence of a large amount of gas during the phase transition, result in significant volume changes, and therefore, despite their large heat of phase change, they are rarely chosen in practical applications.
II. Application Areas of Phase Change Materials
The development of phase change energy storage materials has gradually entered the practical application stage, mainly used for controlling reaction temperatures, utilizing solar energy, and storing waste heat from industrial reactions. Low-temperature energy storage is mainly used for waste heat recovery, solar energy storage, and heating and air conditioning systems. High-temperature energy storage is used in heat engines, solar power plants, magnetohydrodynamic power generation, and artificial satellites. Injecting these materials into textiles can create lightweight clothing with excellent thermal insulation. They can also be used to make insulated cups that retain heat longer than ordinary ceramic cups. Asphalt or cement pavements containing this phase change material can prevent roads and bridges from icing. Therefore, it has broad application prospects in engineering insulation materials, medical and healthcare products, aerospace equipment, military reconnaissance, and daily necessities.
(I) Application of Phase Change Materials in the Pharmaceutical Industry Many medical electronic therapeutic devices require constant temperature operation, necessitating the use of temperature-controlled heat storage materials to regulate the temperature and ensure the instruments operate within permissible limits. A Japanese patent reports the use of a mixture of NaSO4·10H2O and MgSO4·7H2O as a phase change material for temperature control in instrument rooms, maintaining a room temperature of approximately 25°C. Special instruments can also be encased in heat packs made of phase change materials to maintain their operating temperature. In recent years, a type of heat pack has emerged in the domestic market. Its phase change material is a hydrated salt with a phase change temperature of around 55℃. A metal sheet is used as a nucleation seed material; when the metal sheet is squeezed, its surface becomes a crystal growth center, resulting in exothermic crystallization. Combined with certain traditional Chinese medicine bags that promote blood circulation, it achieves a therapeutic effect, showing some efficacy in treating diseases such as rheumatoid arthritis.
(II) Application of Phase Change Materials in Data Storage
PCM is a high-performance, non-volatile memory based on chalcogenide glass. This compound has a crucial characteristic: its resistance changes when it moves from one phase to another. The crystalline phase of the material is a low-resistance phase, while the amorphous phase is a high-resistance phase. Phase transitions are achieved by applying or removing current. Unlike traditional NAND-based non-volatile memory, PCM devices can achieve virtually unlimited writes. Furthermore, PCM devices offer advantages such as short access response time, byte addressability, and random read/write capabilities, making it one of many storage technologies touted as a "future-changing" technology.
In 2017, a research team led by Song Zhitang, director of the Shanghai Institute of Microsystem and Information Technology, achieved a major breakthrough in novel phase-change memory (PCM) materials. They innovatively proposed a design concept for high-speed PCM materials, namely, achieving high-speed crystallization of PCM materials by reducing the randomness of nucleation within amorphous PCM films. Using a 0.13µm-CMOS process, Sc-Sb-Te-based PCM devices achieved high-speed reversible write-erase operations of 700 picoseconds with a cycle life greater than 10⁷ cycles. Compared to traditional Ge-Sb-Te devices, their power consumption was reduced by 90%, while maintaining comparable data retention over ten years. In 2018, memory chip manufacturer SK Hynix began producing PCM-based 3D crosspoint memory. SK explained that this 3D crosspoint memory cell, used in SCM, is made from sulfide-based PCM materials. Recently, IBM research showed that machine learning capabilities can be accelerated a thousandfold by using analog chips based on PCM. An IBM blog revealed that IBM is establishing a research center to develop next-generation AI hardware and explore the application potential of PCM memory in the AI field.
