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For smart hardware, wearable electronic devices, smart phones and other electronic products, batteries seem to be the key bottleneck: the short duration, the need for repeated charging and other issues seriously affect the user experience.

In order to solve the above problems, besides improving the performance of the battery itself, there is another way: using a new way of energy collection and supply. Previously, the author introduced the "self-powered" technology. Among the many ways to realize self-power supply schemes, one is Thermoelectric power generation, also known as "thermoelectric" power generation.

First, let's briefly understand the thermoelectric effect. It refers to the process of voltage generated by temperature difference in special materials. Generally speaking, when one end of the material is hot and the other end is cold, the charge carrier will move from the hot end to the cold end, forming an electromotive force, thus generating a voltage. This material only needs a temperature difference of less than one degree Celsius to generate the detected voltage.

Thermoelectric effect is a thermoelectric phenomenon caused by the contact of different kinds of solids. It has three main effects: Seebeck effect, Peltier effect and Thomson effect. In order to make it easier for you to understand the innovations we are going to introduce today, we will focus on the Sebeck effect.

Seebeck effect, also known as the first thermoelectric effect, refers to the thermoelectric phenomenon of voltage difference between two substances due to the temperature difference between two different conductors or semiconductors. It is generally stipulated that the direction of thermoelectric potential is: the current at the hot end flows from negative to positive. In the circuit composed of two metals A and B, if the temperature of the two contact points is different, the current will appear in the circuit, which is called thermal current. The corresponding electromotive force is called thermoelectric force, and its direction depends on the direction of temperature gradient.

The effect was discovered by French scientist Sebeck in 1821. Later, it was found that thermoelectric potential has two basic properties as follows: first, the law of intermediate temperature, that is, thermoelectric potential is only related to the temperature of two nodes, and has nothing to do with the temperature of the conductor between the two nodes. Secondly, the law of intermediate metals, that is, the thermoelectric potential formed by the contact of conductors A and B, is independent of whether the third metal C is connected between the two nodes. As long as the temperature T1 and T2 of the two nodes are equal, the temperature difference EMF between the two nodes is equal. Because of these two properties, thermoelectric phenomena will be widely used nowadays.

Thermoelectric (TE) materials will play an increasingly important role in the future development of science and technology. For example, it can be made into jewelry, using human body heat to power implantable medical devices such as blood sugar monitoring or heart monitoring equipment; it can also be used in cooking pots, using the heat generated by it to charge smartphones; or it can be used in electric vehicles to convert engine heat into electricity; it can also be used in flying. Machines use the temperature difference between the engine room and the external cold air to generate more energy. Finally, power plants can use this material to obtain more electricity from waste heat.

Although scientists have explored some applications of thermoelectric materials, most of them are still confined to high-temperature equipment. In addition, materials currently used for thermoelectric power generation, such as cadmium, telluride, mercury-based materials, are toxic.

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Over the years, researchers have been looking for suitable materials to ensure that they are non-toxic and environmentally friendly, and that they can produce more electricity. Recently, researchers from Osaka University, Japan, and Hitachi have developed a new TE material, which improves the power factor at room temperature. Their research results, published in the Journal Physica Status Solidi RRL, will help thermoelectric materials break through the high temperature limit and apply to a wider range of fields.

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TE materials have thermoelectric effects: when heat is applied to one side, an electric current begins to flow. On the contrary, a temperature gradient will be formed when external current is applied to the equipment. For example, one side becomes hotter than the other. TE materials can be used in power generators (given heat sources) or refrigerators (given power sources) by converting heat to current.

The ideal TE material should have high conductivity, allowing current to flow through, and low thermal conductivity to prevent the temperature gradient from disappearing at night. Power generation performance mainly depends on "power factor", which is proportional to conductivity and "Sebeck effect".

Sora-at Tanusilp, co-author of the article, said: "Unfortunately, most TE materials are usually based on rare or toxic elements. To solve this problem, we combine silicon (a commonly used TE material) with ytterbium to produce ytterbium silicide [YbSi2]. There are several reasons why we choose ytterbium from many materials. First, its compounds are good conductors. Secondly, YbSi2 is non-toxic. Furthermore, this compound has the characteristics of valence fluctuation, which makes it an excellent TE material at low temperature.

The first advantage of YbSi2 is that Yb atoms occupy mixed valence states, +2 and+3. This fluctuation, also known as "Kondo resonance", maintains high conductivity of metalloids at low temperatures, thus enhancing the Sebeck effect and thus increasing the power factor.

Secondly, YbSi2 has an unusual hierarchical structure. Yb atoms occupy crystal planes, similar to pure Yb metals. Among these crystal planes, silicon atoms form hexagonal sheets, similar to carbon atom sheets in graphite. This prevents heat from conducting through the material, thus maintaining a low thermal conductivity and a temperature gradient. Researchers believe that heat conduction can control nanostructures