| Studies about tetrapod ZnO are now facing many challenges, that the growth mechanism and other theory questions still remain in dimness, that high pure zinc powders are unreasonably demanded in the present techniques and fussy processing of zinc powders is a prerequisite in some cases, and, that mass production is difficult to achieve and higher costs thus hinders the whisker from wide utilization in many fields. From these considerations, this study aimed at investigating and understanding its growth mechanism and researching the conditions of preparing tetrapod ZnO using waste galvanizing zinc on the base of the knowledge obtained from the former, so as to develop a new technique of preparing tetrapod ZnO with own intellectual property.Prior exploration were focused on both thermodynamics and kinetics of the system of liquid zinc — evaporation — vapor oxidation — crystallization. The thermodynamics revealed that zinc vapor was easier to be condensed to form droplets by inert gases of double atoms such as nitrogen round temperature of its boiling point than the melting point. The partial pressure of zinc vapor in the system could be modified by the equilibrium of zinc evaporating below the boiling point of zinc. The kinetics manifested that the evaporating rate was determined by not only the temperature but also the gaseous composition above the molten zinc. The zinc evaporating rate increased with the raise of oxygen content inthe gaseous phase and advanced linearly in all the different atmosphere.Criterions stem from physical and chemical principles of VS and VLS growth model were elicited for practical judgement. As whiskers grow in the VS model, gaseous atoms impinge on the side faces of the whiskers and diffuse to the whiskers tip. Therefore, the whisker growth rate is determined by the atoms balance equation. It was concluded that the elongation rate was constant during the whisker growth. While in the VLS model, mass diffusion from liquid to solid is the key factor of whisker growth and the relationship between elongation rate V and whisker diameter d can be deduced from Gibbs—Thomson effects. It was deduced that V1'2 -\ld changes linearly in the VLS model.Waste zinc and zinc powders and high pure zinc were used as raw materials and zeolite and Bi2O3 as catalyst in experiments investigating effects of conditions of oxidizing zinc vapor at high temperature on the crystal morphologies of ZnO and relations among various morphologies. Results shown that there were five typical morphologies—amorphous, granular, needle, tetrapod and multipod-like ZnO, in spite of the differences of raw materials and no matter catalyst was added or not. All morphologies of ZnO crystals depend on the temperature, oxygen partial pressure and the total gas flow rate when oxidizing zinc vapor. Therefore, ZnO crystal morphologies can be effectively controlled by adjusting the preparing conditions.Further attention was paid on what is the reason of morphologies varying with preparing conditions. It was found that the kinetics of oxidizing zinc vapor at elevated temperature varied with the atmosphere. It was speculated that the dynamic equilibrium between zinc droplets and atomized zinc is the cause of kinetics change. When temperature and oxygen in the system are higher or lower, no zinc droplets formed in zinc vapor and atomized zinc react directly with oxygen. The homogeneous oxidation reaction advances linearly to produce ZnO with morphologies of amorphous, granular and needle-like. As oxygen turned to moderate or low, zinc droplets forming in the zinc vapor made heterogeneous nucleation take place on the surface of the droplets, so, the reaction kinetics proceeded parabolically and products of tetrapod and multipod-like ZnO appeared. So, different morphologies are obtained under different oxidation conditions because of the difference of nucleation and growth process of ZnO crystals. It was obvious that the key factor to produce tetrapod ZnO is to form zinc droplets in the vapor. Zinc droplets can be formed by condensation only at proper temprature, oxygen content and the gas flow rate.The elongation rate at different growth stages of tetrapod ZnO was determined by "Time mark" experiments and morphologies of products at different atmosphere and growth stages were examined by scanning electron microscope. Results shown that the elongation rate of tetrapodZnO was governed by spiral growth and the crystallization was via the VLS model. The growth of tetrapod ZnO was found to be consisted of stages:(l)atomized zinc evaporating from the molten zinc and entering the gaseous phase above;(2)atomized zinc being condensed by inert gas and forming tiny zinc droplets;(3)oxygen absorbed on the droplets and oxidation reaction taking place;(4)heterogeneous nucleation proceeding on the surface of the droplets and zinc diffusing outwards as whiskers growing. Morphologies of the products are thus determined by the dimension of the zinc droplets. Smaller droplets formed under mild oxygen partial pressure, will be oxidized to form tetrapod ZnO, while small droplets formed under lower oxygen pressure were oxidized to form mutipod ZnO.The process of oxidizing zinc droplets was that the zinc atoms inside droplets diffuse outwards and generate growth steps on the side and terminated faces of the whisker. The elongation of whiskers continued until zinc atoms inside the droplets were depleted. Although, VS crystallization, which stood by from the beginning to the end but had little effect on the elongation growth, took place at side faces of the whiskers and consequently made the whiskers radius increase.The kinetics of zinc droplet oxidation is divided into two stages. First, when t/t05 < 1.5, the kinetics proceeded as:[1 - (1 - a)1'3} = (0.0249 ~ 0.0971Vand then, when / / /0 5 > 1.5, the kinetics followed the equation below :[1 -\a-(l-cO2"] = (2.347- 9.230)xlO'3tOn the base of knowledge obtained from studies above, new technique of preparing tetrapod ZnO was developed. The purification must be accomplished at the same time when tetrapod ZnO was prepared directly from the waste zinc. For this purpose , the behavior of impurity elements at the oxidation conditions were examined to understand the effects on the quality of products. Elemental As and Sb access in the products while their compounds remain in the residue. Elements of Ok Co> Iik Ni et al, have little effects on the products for their high meting temperature and negligible content. Fe content in whiskers is in the way of mechanical carry out by the zinc evaporating and the positive effects on the activity coefficient of Fe-Zn system, which can be controlled by the oxidizing atmosphere and the evaporating rate of the waste zinc. Pb content in whiskers is for the evaporation of Pb element itself and the positive effects on the activity coefficient of Pb-Zn system, which can be restrained by the modification of the oxidizing atmosphere.The optimized conditions of directly preparing tetrapod ZnO by waste zinc were concluded below: temperature of 850-950"C; oxygen partial pressure of 8-12 v%; total gas flow rate of 140-260 L.h'1; waste zinc evaporating rate being ^93%. Characterization of the products was conducted by means of SEM, ICP-AES, EDS and IR. Results shown thatthe mophology of products is uniform tetrapod ZnO with perfect crystallization of hexagonal wurtzite; length of needles is 80-130//m and ratio of radius over length is 25-30; ZnO content is ^99.96% and yield of whiskers is ^92%; the quality of products can rival the whiskers produced by pure zinc powders.Features of the new technique are listed: free of catalyst, no special restriction on the particle size and the purity of the raw material after crashing, the whisker fabrication is under atmosphere pressure and moderate temperature; mass production of whisker is easy to achieve for the simplicity of the process and equipments; impurities are separated by controlling preparing conditions at the same time when whisker grow.
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