As a new synthetic material, MAX combines the excellent properties of both metal and ceramic materials, and has good application prospects in fields such as high-speed ferroelectric bows, high-temperature insulation covers, and wear-resistant coatings. However, the preparation of such materials has always been a challenge.

Huang Qing, a researcher at the Ningbo Institute of Materials, Chinese Academy of Sciences, led a team to propose a new method that can create many new MAX phase and MXene materials that cannot be synthesized through conventional pathways, similar to the "Twisting Rubik's Cube". This not only broadens the application range of such materials, but also brings infinite imagination to material synthesis. On March 17th Beijing time, the research results were officially published in the journal Science.

The periodic table summarizes the types of elements that make up MAX phase and MXene. Light blue represents the M-site element, earthy brown represents the A-site element, black represents the X-site element, green represents the terminal group element, and the circled elements are the elements that have been experimentally verified in the article. Source: Ningbo Institute of Materials, Chinese Academy of Sciences

Special synthetic materials

What kind of chemical reaction will occur when metals and ceramics "strongly combine"?

As a special artificial synthetic material, MAX phase materials have always been one of the research hotspots in the field of materials. Huang Qing introduced that M represents pre transition metal elements; A represents the main group elements of IIIA and IVA; X represents carbon, nitrogen, or boron. M. The three types of elements in A and X can be arranged and combined differently to form various MAX phase materials.

Thanks to the unique nano layered crystal structure, this type of material can conduct electricity and heat like metal, is soft and easy to process, and has high toughness; Similar to ceramics, it has characteristics such as oxidation resistance, high temperature resistance, and radiation corrosion resistance. This has shown great application prospects in fields such as high-speed railway pantographs, high-temperature heating elements, turbine blades, high-temperature insulation covers, and wear-resistant coatings.

If element A is removed, the MAX phase material will be derived into a new type of material MXene. It is understood that the atomic arrangement of MXene is similar to that of graphene, and it has great potential for application in fields such as optoelectronic devices, electrochemical energy storage, electromagnetic shielding, surface catalysis, and separation membranes.

However, in the three types of raw materials M, A, and X, some elements are not compatible. Allowing them to coexist peacefully and preparing high-quality materials with specific functions is a challenge.

Schematic diagram of structure editing strategy for layered transition metal carbides assisted by "chemical scissors" Source: Ningbo Institute of Materials, Chinese Academy of Sciences

Twisting Rubik's Cube to Prepare New Materials

The composition and structure of chemical elements determine the properties of materials.

In order to meet the application needs of different fields, scientists hope to accurately regulate the microstructure of MAX phase and MXene materials, and prepare new materials with specific functions.

As early as 2019, Huang Qing's team proposed a green preparation technology and successfully synthesized zinc MAX phase materials for the first time. The previous work was actually using an oxidizing 'chemical scissors' to etch elements between layers, that is, to reduce the elements we need to regulate, "Huang Qing said.

Can elements be reassembled after being etched? To verify this hypothesis, Huang Qing's team conducted multiple experiments. Finally, they discovered that metal, a reducing "chemical scissors," can reassemble two-dimensional MXene materials into three-dimensional MAX phase materials.

Huang Qing explained that MAX phase materials are like building blocks, composed of three types of elements: M, A, and X, which are sequentially spliced from left to right. In the past, to cut the middle element A, scientists needed to break down three building blocks, equivalent to crushing the entire chemical structure, in order to "extract" the middle element A.

And their proposed new strategy is similar to the "Twisting Rubik's Cube", which slightly twists the middle layer of the three-layer Rubik's Cube without damaging the original structure, and element A is stripped off. By twisting the Rubik's Cube block representing the new element back to its original position, the MAX phase material achieves "isomorphic substitution".

Huang Qing is guiding graduate students. Source: Ningbo Institute of Materials, Chinese Academy of Sciences

Bringing endless imagination to material research

The continuous transformation and combination of chemical "magic cubes" have brought endless imagination to material research.

With the help of the "chemical scissors" assisted structural editing method, Huang Qing's team has prepared a series of new MAX phase materials that cannot be synthesized by conventional methods.

Among them, the introduction of non-traditional A-site elements, such as magnetic elements and precious metals, is expected to expand them from high-temperature structural fields to functional applications, such as magnetism, optoelectronics, catalysis, superconductivity, etc.

Using this method, Huang Qing's team has also obtained a series of new MXene materials, which are expected to promote the further application of MXene in catalysis, energy storage, electromagnetic shielding, and other fields.

"This opens a new way for material synthesis, which is a directional success." Chai Zhifang, an academician of the CAS Member and chief scientist of the Advanced Energy Materials Engineering Laboratory of Ningbo Institute of Materials, Chinese Academy of Sciences, explained that this method is like making sandwiches according to everyone's "taste". Different ingredients can be added to two pieces of bread, such as chicken, fish fillets, lettuce, etc., so that everyone can take what they need.

Many important materials that have received much attention can be synthesized using this method, and I believe it will receive more and more attention and applications in the future, "Huang Qing said.