Metal and Metal Hydroxides Based Amorphous Materials from Bottom up Strategies


Student thesis: Doctoral Thesis

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Awarding Institution
Award date29 Aug 2022


Atomic structures determine the materials’ properties and performances. Amorphous materials possess disordered atomic structures, showing unique physical and chemical properties compared to their crystallized counterparts. Synthetic methods of amorphous materials, particularly metallic glasses, are generally based on top-down strategies. For example, a common method for synthesizing amorphous inorganic materials is ball milling, which causes amorphization of materials by strong collisions between bulk crystallized materials with metallic or ceramic spheres. Another example is to produce metallic glasses from melts by rapid quenching at a very high cooling rate. As for monatomic metallic glasses, an extremely high cooling rate is required, making it a difficult task---to date, only a few types of monatomic metallic glasses have been synthesized.

High-entropy materials exhibit superior performance in many applications due to their unique atomic structures. The current study of high entropy materials more focus on alloys, in which usually five or more metal elements with equal molar ratio are mixed. Recently, high entropy ceramics, such as oxides, nitrides, and borides, have attracted much attention. For high entropy ceramics, the materials reported to date are generally crystalline, whereas amorphous ones are rarely studied.

This thesis explores synthesis of novel amorphous monatomic metals and high-entropy hydroxides using bottom-up strategies. Electrochemical discharge reduction successfully achieved amorphous monatomic metal including Sn and Ag. Amorphous high-entropy hydroxides have been fabricated through the co-precipitation method. Due to the synergistic effects of high entropy and amorphous states, the obtained amorphous high-entropy hydroxide of NiFeCoMn(OH)x displays a narrowed bandgap of 1.1 eV. Moreover, the synthesized amorphous NiFeCoMn(OH)x gel possesses good molding ability, directly applicable as ink for 3D printing. After drying at room temperature, monolithic ceramic objects are obtained, featuring good mechanical performance with hardness/reduced elastic modulus (H/Er) of 0.8/20 GPa.

Chapter I gives introduction of definition, classification, synthetic methods of amorphous materials.

Chapter II introduces materials, experimental instruments and experimental methods mentioned in this thesis.

Chapter III reports a new synthetic method of amorphous monatomic Sn spheres and Ag nanoparticles based on anodization methods. The electrochemical discharged reduction mechanism is proposed: Sn2+ ions are reduced into Sn0 in a very short time. HRTEM and SAED characterization reveals their amorphous nature. Meanwhile, TEM-EDX characterization exhibits trace oxygen contents (0.23 at%) in the Sn spheres, confirming their high purity. This method offers new route for synthesizing amorphous monatomic metals using the bottom-up strategy.

Chapter IV reports the amorphous high-entropy hydroxides synthesized through coprecipitation in water solutions. PXRD and HRTEM characterizations confirm the total amorphous state of the obtained NiFeCoMn(OH)x. Meanwhile, TEM-EDX characterization proves the even distribution with equal molar ratio of different metallic cations in the material, which is consistent with the high-entropy definition. Due to the synergetic effect of amorphous state and high entropy, amorphous high-entropy NiFeCoMn(OH)x possesses a narrowed bandgap of 1.1 eV. As a result, the amorphous high-entropy NiFeCoMn(OH)x exhibits a good photothermal conversion efficiency, which is comparable to that of black TiO2.

Chapter V reports synthesis of monolithic ceramic at room temperature consisting of amorphous high-entropy hydroxides. The obtained precursor NiFeCoMn(OH)x gel exhibits good molding ability, which can be molded into complex shape through 3D printing. Moreover, after drying at room temperature, the obtained monolithic ceramic shows good mechanical performance with hardness/reduced Young’s modulus of 0.8/20 GPa.

Chapter VI gives a conclusion and outlook for this thesis.