Recently, researchers have grown tiny "snowflake" zinc crystals using gallium (Ga), a liquid metal with a low melting point, as a solvent. The key to their success is that the researchers applied a voltage to the liquid metal solution while vacuum filtering it. The resulting metallic crystals exhibit a variety of complex morphologies and enduring symmetries, some of which closely resemble snowflakes in nature. This strategy can be applied to a variety of metals and alloys, producing metal crystal particles of various morphologies.
The metal dissolves readily in liquid Ga at high temperatures and crystallizes on cooling. However, liquid metals are not volatile, and the high surface tension of Ga prevents the precipitation from separating from the metal solvent, making extraction of these crystals very difficult. Researchers have previously demonstrated that electrochemical actuation can tune the surface tension of liquid metals from a natural value to near zero. So, they first dissolved the solute metal (such as Zn) in Ga at high temperature to form a uniform liquid alloy. Subsequently, it is cooled to room temperature to supersaturate the solute metal and precipitate out in crystalline form. Immediately after applying a voltage, the surface tension of the liquid metal becomes low. After vacuum filtration with NaOH solution, almost all liquid Ga can pass through the porous membrane. After washing the remaining Ga in the residue, pure metal crystals can be obtained.
When metal Zn is used as a solute, it is very easy to grow into a crystal form similar to snowflakes. Hexagonal Zn nuclei are first formed during the cooling process of the Zn-Ga system. AIMD simulations also show that the Zn(0001) plane with good structure is the dominant crystal plane for the crystallization process. The influence factors such as secondary nucleation, plane instability and the difference in growth rate on different crystal planes make Zn crystals further form more complex structures, and thus lead to the final snowflake morphology.
Solute concentration, crystal growth time, temperature and pressure all have an effect on snowflake crystal morphology. By controlling the variable method, the researchers summarized the characteristic shape and relative size of the crystals (350 °C initial temperature and ambient pressure) at Zn concentration ranging from 5 to 20 wt%, and crystallization time of 1 day, 2 days, and 10 days. At lower Zn concentrations, a dendritic structure tends to form, while at higher concentrations a fractal structure dominates.
Furthermore, this method can be extended to most metals dissolved in Ga, such as Sn, Bi, Ag, Mn, Ni, etc., and even ternary systems. The crystal growth in the binary system can be divided into two categories. In the first category, including Sn-Ga, Bi-Ga and Zn-Ga systems, the solute metal phase is completely separated from the liquid Ga during the crystallization process to form a single metal crystal. The other category includes Ag-Ga, Mn-Ga, Ni-Ga, Cu-Ga and Pt-Ga, and solute metals can form intermetallic crystals with Ga. Surprisingly, the crystallization of the ternary Al-Mn-Ga system produced an Al11Mn4 intermetallic phase consisting of only two solute metals, which calculations indicated had a relatively low Gibbs free energy. This approach greatly expands the crystal library, revealing the morphological and compositional diversity of metallic crystals.
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Reference
- Liquid metal synthesis solvents for metallic crystals
Shuhada A. Idrus-Saidi, Jianbo Tang, Stephanie Lambie, Jialuo Han, Mohannad Mayyas, Mohammad B. Ghasemian, Francois-Marie Allioux, Shengxiang Cai, Pramod Koshy, Peyman Mostaghimi, Krista G. Steenbergen, Amanda S. Barnard, Torben Daeneke, Nicola Gaston, Kourosh Kalantar-Zadeh
