Synthesis And Corrosion Behavior Of Ti₂AlN Ternary Ceramics

The MAX phases are layered ternary compounds. A new family of two-dimensional materials called MXene are produced by the etching out of the A layers from MAX phases. MXenes are expected to be good candidates for a host of application in Supercapacitors, lithium-ion battery. Ti2A1N is one typical member of nitrides MAX phases, therefore, it is necessary to develop rapid and low-cost ways to prepare high purity and small size Ti2A1N ceramic in mass scale, which is the foundation of its corrosion. In this work, the new method for synthesizing Ti2A1N and its corrosion behavior in solution were investigated.The powder mixture of Ti, Al and TiN was used as the raw materials with a designed composition. The mixture was firstly cold-pressed into cylindrical discs and directly put into an air furnace which was preheated to 650℃～850℃. After holding for 2-5 min, the reacted discs were taken out and cooled down quickly to ambient temperature. The structures and compositions of the as-prepared products were characterized by XRD, FESEM and HRTEM. The effects of starting composition, TiN particle size, preheating temperature, the compacts’volume (height, diameter) and holding time of the prefabricated discs on compositions, microstructure of as-prepared H2A1N were investigated, and the formation mechanism for Ti2A1N was also proposed. Results show that purity of Ti2A1N increase in direct ratio with Al content and later decrease with continuously increasing amount of Al. The TE could be initiated at 650℃, but several intermediate phases present in the samples, even the preheating temperature increased to 700℃ or higher, TiN phase was still residual and can not completely eliminate. Futhermore, the larger volume disc results in more heat energy, but it takes more time for the heat energy transfer in the whole disc in a larger one and the whole TE process in compact seems occurred quickly and lasts only tens of seconds. Besides, pressure, holding time have little effect on the Ti2A1N samples. Herein, the high purity Ti2AlN has been synthesized by the thermal explosion with the start molar ratio Ti:Al:TiN=1:1.05:1 at 700℃ with a pressure of 2 MPa and the thickness of prefabricated discs less than 6 mm. The whole process of synthesizing Ti2A1N could be divided into two stages: (i) preheating stage, Al would firstly melt and spread and formed Ti-Al melt began to cover the surface of the TiN particles, (ii) Dissolution and Nucleation-growing stage, TiN particles dissolved into the Ti-Al melt, and ternary phase Ti2A1N began to precipitate from the melt growing into a layered morphology.The corrosion behavior of Ti2A1N in hydrofluoric acid solution and different corrosive media, such as DMF, HC1, CH3COOH, CH3COCH3, etc. by wet milling at room temperature were investigated. The structures and compositions of the corrosion products were characterized by XRD, FESEM. The results show that Ti2A1N powders were placed into the 2% HF solution for 2 h, bubbles were observed. This phenomenon strongly suggests a chemical reaction occured. The microstructure of the samples is quite thin about 2-5 nm. When the concentration of HF solution is too low or too high, the reaction time is too short or too long, Ti2A1N powders can not be peeled off or excessive corrosion lead to further oxidized. And by ball milling, the Al atom layer can not be selectively etched, but it can make the three-dimensional Ti2A1N to two-dimensional nanosheets (with the thickness less than 10 nm).Corrosion behavior of Ti2A1N synthesized by thermal explosion was investigated under hydrothermal conditions in HCl, H2SO4, HNO3, LiOH and NaOH. The structures and compositions of the corrosion products were characterized by XRD, SEM. The study indicates that the corrosion resistance of the Ti2A1N in acid weaker than in basic solution. During the corrosion process under hydrothermal conditions, Al atoms in Ti2AlN first react with  or OH", leaving TiN layers with dangling Ti atoms on their top and down surfaces. Then the active dangling Ti atoms are prone to form monomers. In acid solution, effect by SO42-, Cl-,NO3- anion these polymers grow to different crystal forms of TiO2; in basic solution, the polymers formation to titanate nanowires.