Synthesis Of TiN Nano Powders By Arc In Liquid Nitrogen

Nano Science and Technology (Nano-ST) is a new science and technology, which was born in the late of 80's and develops quickly. Nano-ST studies the special phenomena and function of materials whose size range is from 0.1nm to 100nm. Nanomaterials are the branch of Nano-ST which are active and abundant on research contents. The system of nano-materials has many special characteristics and new laws, which make people realize the field would be hot in materials science research in the new century. And its development will bring new chances to the academic research of physics, chemistry, materials, biology and medicine etc. Nanomaterials have many new natures which are different from the normal materials for their small size, big specific surface area and quantum dimension effect. Nano-powders have attracted much attention to many important potential applications in optics, electricity and electronics, magnetism, mechanics and so on. Especially, nanomaterials made from nano-powders are the functional materials with manifold excellent properties and have broad market potential. There are gas-reaction, liquit-reaction and solid-reaction methods reported to prepare nano-powders. These methods are the base for preparation of other nanomaterials. There is a closely relation between the methods and the methods to prepare other nanostructures, such as one or two or three dimension nanomaterials and nano composites. Titanium nitride has been widely used to produce super-hard coatings because of its low friction coefficient, extremely high hardness and abrasion resistance. TiN nano-powders can be added into ceramic matrices to improve the hardness, the fracture toughness and the bend strength. Several methods such as gas condensation, chemical vapor deposition (CVD), plasma torch processing, electro-explosion of wire, mechanochemical processing and so on, have been reported to synthesize crystalline of titanium nitride (TiN) nano-powders. Both gas condensation and chemical vapor deposition methods require expensive vacuum systems and/or a chemical reaction chamber. Furthermore, such methods are currently limited to small batch production. There are several non-vacuum based methods including plasma torch processing, electro-explosion of wire and mechanochemical processing. Production yields are high but cost is also high for the first two methods. While the last one is relatively easy but can lead to pollution on the particles from abraders and the particle components are uneven as well. Therefore, there is an increasing need to develop a simple and cost-effective process that requires inexpensive equipment. This thesis puts forward a novel method to synthesize TiN nano-powders by a direct current arc in liquid nitrogen to meet the above requirements. This method need neither the vacuum system nor inter gas. It involves the application of low-cost liquid nitrogen and the simple experimental device designed by ourselves. Liquid nitrogen can provide the protecting and synthesizing condition around the arc. And also its cooling can restrict the powder growth when the tungsten (W, as cathode) is used to evaporate the titanium metal. The obtained TiN powders will flow into the collection device along with the nitrogen gas automatically under the protection of it without application of cooling wall. In the thesis, TiN nano-powders were synthesized in liquid nitrogen by arc heating, the effect of the energy of the arc (arc current) on the size of powders was studied and physical and chemic changes in the process of synthesizing TiN nano-powders and the mechanism were analyzed. The mean size, the size distribution, the morphology and the component of TiN nano-powders were analyzed by scanning electron microscope (SEM), transmission electron microscope (TEM) and X-Ray diffraction (XRD). Analyzing results showed that the obtained TiN powders are cube or cuboid and have integrity of geometry but in congregated state. The larger the current, the bigger the size of TiN nano-powders. When the current is 40 A, the mean sizeis 30 nm; when the current is 80 A, the mean size is 70 nm; when the current is 120 A, the mean size is 130 A respectively. Because increasing current makes the temperature of molten pool on the surface of Ti specimen increase and Ti vaporize, the steam pressure increases and the density of Ti steam gets bigger. TiN nucleates in denser steam and particles collide in higher probability. At the same time, increasing current enlarges the arc zone. This makes the distance of particle movement longer and the collision probability becomes higher. As a result, the size of particles is enlarged and the optimized current is between 40 A and 60 A from experiments. There are physical and chemical changes in the process of synthesizing TiN nano-powders in liquid nitrogen by arc heating. Physical change includes solid Ti becoming into liquid and furthermore Ti is vaporized into atom and ion states. Liquid nitrogen is gasified simultaneously. Moreover, molecules, atoms and ions of nitrogen exist in the arc zone and then enter into the molten pool by absorption, solution and diffusion. The chemical change is [Ti]+[N] →TiN. The mechanism of synthesis of TiN nano-powders has two models. They are the model of TiN nano-powder nucleation in arc zone and the model of reaction evaporation process of TiN nano-powders in molten pool. The first case is that Ti vapour reacts with molecules, atoms and ions of nitrogen to synthesize TiN nano-powders in arc zone. The second case is that atoms and ions of nitrogen enter into Ti molten pool and synthesize TiN on the surface of molten Ti. TiN was vaporized because of the arc effect. TiN nano-powders are synthesized when TiN powders nucleate, grow up and cool in arc zone. There was a layer of TiN on the surface of Ti substrate after experiment. In order to prove the second model, an experiment was designed which the arc was ignited on the surface of Ti substrate for a short time. The result indicated that atoms and ions of nitrogen can enter into Ti substrate and the molten pool by absorption, solution and diffusion and then react with liquid Ti or solid Ti around molten pool at high temperature to synthesize TiN. With the continuous burning of the arc, TiN...