New Material: Thermoelectric Hydrogel Can Be Applied To All Kinds Of Clothes

The utilization efficiency of heat energy is only about 30%, and most of the heat will be dissipated in the environment in the form of waste heat, of which 2/3 of the waste heat temperature is lower than 200 ℃. Effective recovery of low-grade waste heat is of great significance for sustainable development.

Thermoelectric hydrogel plasma thermoelectric materials, represented by quasi solid gel, have attracted much attention due to their thermal voltage of 2-3 orders of magnitude higher than that of electronic thermoelectric materials.

What are the good properties of thermoelectric hydrogel materials? What applications are there in the future? With these questions, the reporter interviewed relevant experts.

　 A material with excellent thermoelectric properties

Thermoelectric hydrogel is a kind of material with excellent thermoelectric properties, which is widely used in thermoelectric converters and heat sinks. Because of its flexible and tensile properties, it can be made into materials with different hardness, thickness and color through artificial design, and even can be made into films conformally fitted with the skin surface.

Professor Zhang Hulin, School of Electronic Information and Optical Engineering, Taiyuan University of Technology, introduced that when the thermoelectric hydrogel material is placed in an environment with temperature gradient, the thermo voltaic effect of redox pairs in the material will form an electrochemical potential difference at both ends of the material. If the upper electrode is laid, the electrode potential difference will be generated at both ends of the device, that is, the voltage difference will be formed.

There are many methods to prepare thermoelectric hydrogels, which can be roughly divided into physical cross-linking method and chemical cross-linking method. The specific preparation methods include sol-gel method, electrophoresis method, polymerization method, etc.

Yang Hang, a graduate student of Taiyuan University of Technology, introduced that the commonly used preparation method of her research team at present is the sol-gel method, that is, select the appropriate solvent to dissolve the required thermoelectric materials to form a colloidal solution. Under appropriate conditions, the colloidal solution can be converted into thermoelectric hydrogel by heat treatment or other methods.

"Polymerization is also a chemical method. By adding cross-linking agents, initiators and other chemical reagents to the monomer solution, free radical polymerization can take place to form thermoelectric hydrogels. The electrophoretic method is to electrify the colloidal solution, so that it can polymerize under the action of an external electric field." Yang Hang said.

　　 It is widely used in daily life

Thermoelectric hydrogel materials have many applications in daily life, for example, thermoelectric generators are devices that use thermoelectric effects to convert heat energy into electrical energy. In thermoelectric generators, thermoelectric hydrogels as thermoelectric materials can convert waste heat into electrical energy.

Liu Kang, a researcher at Wuhan University, and Hu Xuejiao, a professor, have designed and developed an intelligent thermoelectric hydrogel. The hydrogel film is applied to the heating element. On the one hand, it can directly convert waste heat recovery into electric energy; on the other hand, the water in the hydrogel will quickly evaporate to take away heat and reduce the temperature of the device. At the same time, when the device stops working, the hydrogel film will absorb water from the surrounding air and automatically realize reciprocating utilization.

At the same time, thermoelectric hydrogels can also be used in temperature sensors to measure temperature. Since the thermoelectric properties of thermoelectric hydrogels are closely related to temperature, the temperature change can be determined by measuring the voltage difference of thermoelectric hydrogels. The temperature sensor based on thermoelectric hydrogel has the advantages of fast response and high precision.

Combined with the characteristics of thermoelectric hydrogel, Zhang Hulin's team explored the use of thermoelectric hydrogel to achieve passive wearable motion monitoring, which is also one of the promising application directions of thermoelectric hydrogel.

The flexible edible hydrogel sensitive unit developed by the team can be used to monitor the biological pressure on the baby's body surface. "We attach sensors to baby clothes and skin, and 11 pressure sensors cover the chest, hands, knees, feet, neck, back, wrists, buttocks and other key parts respectively. We collected signals of specific motion modes in advance, such as turning over, holding babies, clapping back and clapping, and developed deep learning algorithms based on these signal characteristics, which can quickly and accurately recognize Don't let the baby be active. " Zhang Hulin introduced.

Although there are relatively few domestic researches on passive wearable motion monitoring, thermoelectric hydrogel materials can be directly applied to skin or clothing, and the sensitive units can be large or small, providing infinite possibilities for passive wearable motion monitoring.

The research group of Professor Ma Rujun of Nankai University has designed a kind of strong tensile thermoelectric hydrogel with high thermoelectric properties based on tensile induced crystallization and thermoelectric chemical effects. This research overcomes the low mechanical strength limit of traditional quasi solid thermoelectric cells, and optimizes the thermoelectric properties, It provides a feasible scheme for designing wearable electronic devices with efficient thermoelectric conversion and flexibility and reliability.

When talking about the development of thermoelectric hydrogel materials, Zhang Hulin expressed full confidence. "In the future, we will continue to improve the stability of signal conversion, develop more suitable signal processing algorithms, and develop a series of passive intelligent wearable devices based on existing research results and our own professional expertise. We believe that industrialization is in the near future," said Zhang Hulin.

　 Linking the new gel can realize the conversion of light, heat and electricity

Recently, Professor Zhang Hulin from the School of Electronic Information and Optical Engineering of Taiyuan University of Technology has prepared a kind of light driven flexible thermoelectric hydrogel. The gel part is composed of polyvinyl alcohol, polydimethylsiloxane polymer network and ferric ion/ferric ion redox pair. This achievement was published in the Journal of Chemical Engineering.

On the basis of traditional thermoelectric hydrogel materials, the team members have prepared a light driven flexible thermoelectric hydrogel patch. This achievement adopts a double-layer structure, the upper layer is composed of polydimethylsiloxane polymer, and doped with silica coated gold nanoparticles, which uses the local surface plasmon resonance effect to achieve photothermal conversion. The lower layer is a film composed of polyvinyl alcohol network and ferric ion/ferric ion redox pair, and doped with polydimethylsiloxane polymer particles to reduce the thermal conductivity. "The wavelength of visible light is about 400 nm to 800 nm. When the overall vibration frequency of the surface electron of gold nanoparticles in the polydimethylsiloxane polymer network roughly matches the visible light, it can resonate with the visible light and absorb the photon energy, realizing the conversion from light to heat." Zhang Hulin explained.

Zhang Hulin introduced that in addition to the organic framework, more than 70% of thermoelectric hydrogel materials are composed of water. In the north and other regions, because of the dry climate conditions, it is easy to make the thermoelectric hydrogel material lose water, thus affecting its thermoelectric conversion efficiency.

"The key to improving the stability of thermoelectric hydrogel performance is to ensure no water loss." Zhang Hulin said that at present, there are relatively few teams engaged in thermoelectric hydrogel material research in China, and the experience available for reference is relatively insufficient. In this research, the team experienced several months of exploration, and reduced the water loss rate of the material by adjusting the internal physical and chemical structure of the material and preventing water evaporation through external packaging.