Preparation Of Cr₂GaC Max Phase And Research On The Rules Of Spontaneous Growth Of Gallium Whiskers

The MAX phases are a new group of layered ternary compounds with the general formula Mn+1AXn (M: early transition metal; A:group A element; X:C or N; n=1,2,3...). The electronic structure study indicates that the M-X atoms connect with strong covalent bonds and ionic bonds, the M-A atoms connect with relatively weak covalent bonds and metallic bonds, while M-M atoms connect with metallic bonds. The unique crystal structure of MAX phases makes it possess a combination of excellent properties of metallic and ceramic materials. Thus the MAX phases have showed potential applications in Li-ion batteries, fuel cell, high-speed train and military industries. However, A element whiskers were found spontaneously grown on the surface of MAX phases, which were similar with the spontaneous growth of general metal whiskers, such as tin whiskers. Naturally, this phenomenon has aroused suspicion about performance stability of MAX phase. In recent years, the research work of A element whiskering phenomenon is still in its incipient stage, and traditional metal whisker growth models and mechanisms, such as compressive stress mechanism, oxidation theory have been borrowed in the research work on MAX phase materials. But these mechanisms were proposed in a specific environment and aimed at a specific material. Up to now, none of these mechanisms could be accepted by the scientific community perfectly.This thesis research was performed based on samples of Cr2GaC MAX phase. The preparation of Cr2GaC MAX phase and the spontaneous growth of Ga whiskers on the surface of Cr2GaC were studied. Firstly, Cr2GaC was prepared using pressureless sintering. Effect of synthesis temperature and Ga content in the Cr/Ga/C mixture on the phase compositions were discussed. It was found that 120 min preservation at 1100℃ can synthesis the Cr2GaC MAX phases, with the main phase of Cr2GaC. With the increasing of Ga content, the impurity phase changes from Cr7C3 phase to Ga elemental phase.On the surface of the ball-milling and cold-press green-bodies of Cr2GaC, phenomenon of mushrooming growth of Ga whisker was observed. The phenomenon obeyed these three rules:1) free Ga element in the sample is necessary for the whiskers growth in the matrix of Cr2GaC; 2) the Cr2GaC grain cleavage plane created by the ball-milling is necessary for the spontaneous growth of Ga whiskers; 3) the environment temperature, such as low temperature, would accelerate the spontaneous growth of Ga whisker. During the experiment, some new phenomenoa were observed, for example, the cluster whiskers, the mismatch between the matrix pore size and the whisker size, denser whiskers on looser zone of the matrix etc. The compressive stress mechanism borrowed from the field of general metal whisker research failed to explain these phenomena. Therefore, a catalysis-based mechanism for the whiskers growth was proposed. Cleavage planes of Cr2GaC created by the mechanical damage during ball milling serves as the catalyst of spontaneous growth of Ga whisker. Free Ga atoms will nucleate in the cleavage planes and grow into whiskers. Ball-milling was believed vital for spontaneous growth of Ga whiskers. In this mechanism, spontaneous growth of Ga whiskers can be divided into four types. The phenomenon such as calotte whisker can be explained perfectly, thus the cleavage plane catalytic mechanism is more convincing in explaining the spontaneous growth of Ga whiskers.On the other hand, main applications of a MAX phase are expected to be in the dense form. And the spontaneous growth of Ga whisker on the sintered Cr2GaC body would obviously affect its mechanical properties, electrical properties, which may jeopardize a system. Hence, Cr2GaC sintered-bodies were preparared and the effect of Ga content and density on the spontaneous growth of Ga whiskers was studied herein. Experimental observation revealed the following.1) free Ga element in the sample is necessary for the whiskers growth in the sintered Cr2GaC body; 2) the greater density of the sintered Cr2GaC body, the more difficult for the Ga whiskers to grow. The cleavage plane catalytic mechanism was applied to explain the spontaneous growth of Ga whiskers successfully. This again experimentally supports the validity of cleavage plane catalytic mechanism. The mechanism proposed herein would provide a new perspective to solve the whiskering problem in MAX phases.