Hygrometry를 이용한 금속산화물 나노분말의 수소환원 반응속도에 관한 연구

Abstract

Fabrication of nano-sized metal powder by hydrogen reduction of ball-milled oxide powders provides a competitive nanopowder technology especially for bulk nanoparticulated material from the viewpoint of a cost-effective process, an environment-friendly by product, mass production and easy material design. Conventionally, optimizing hydrogen reduction process has been conducted by estimating the weight loss and heat-capacity transition during reduction process using thermogravimetry (TG) and differential thermal analysis (DTA). However, the results from TG and DTA experiment due to a tiny volume of powder sample are hardly used to monitor the reduction process for mass production. Hence, it is necessary to find a new method to estimate the hydrogen reduction behavior for mass production. In this study, mass production of various metal nanopowder from hydrogen reduction of oxide nanopowder was considered. Also, in order to optimize hydrogen reduction process conditions for mass production, we tried to trace the behavior of generated vapor during hydrogen reduction process for mass production by in-situ measurement hygrometry. The oxide nanopowders of iron, nickel, tungsten and copper were prepared by blending a-Fe2O3 (99.9%, 1 mm) and NiO (99.9%, 7 mm) powders. Ball milling was performed in a stainless steel attritor at a speed of 300 r.p.m. for 10h (Fe2O3, NiO, Fe2O3-NiO). The powder-to-ball mass ratio was 1:50 with a powder mass of 100 g. Methyl alcohol was added as a process control agent (PCA). After ball milling, the powders were dried for 12h at 333K and sieved down 100 mesh. The ball-milled powders were reduced at various temperature. The behavior of water vapor during reduction was measured by hygrometry study.

[Hydrogen reduction kinetics of single phase nanopowder] i) Case I : Nonisothermal reduction kinetics of Fe2O3 nanopowder In this study the synthesis and related kinetics of Fe nanopowder by hydrogen reduction of Fe2O3 were investigated by a reduction kinetics experiment using hygrometry in real-time. The ball-milled Fe2O3 agglomerate powder of 20-50 nm in grain size, was used for this study. Activation energy for Fe2O3 reduction showed that initially it decreased slightly from 46 kJ/mol to 35 kJ/mol until a reached 0.8 and then it was abruptly lowered to 20 kJ/mol beneath a=0.8. In this study, it was found that the activation energy in range of 0.1≤a≤0.8 decreased slightly due to the change of activation energy with the phase transformation. Also the activation energy in the range of 0.9≤a≤1.0 decreased dramatically, because the residual water vapor in the reactor was not removed after completed reduction. The activation energies obtained from the hygrometry curve seem reasonable to exist approximately in the range of reported data for the hydrogen reduction process in a number of previous work.

ii) Case II : Isothermal reduction kinetics of Fe2O3 nanopowder The kinetics of the reduction of nc Fe2O3 powder with hydrogen were measured in the temperature range from 573K to 813K. The following conclusions can be drawn from this work: ① Comparison of the reduction behavior of nc Fe2O3 (particle size : 20-50 nm) with that of conventional powder (particle size : several hundred nanometers) shows no discernible difference in the reduction rate, although the nc powder has a significantly larger specific surface area. ② The results were checked for the possibility that the reduction of the nc powder might have been inhibited by pore diffusion. This was suspected because of the strong tendency of the nc powder to form agglomerates, the size of which exceeded the particle size of the as-received powder by a factor of ten. It was found that the likelihood of such an influence is rather slim. ③ Comparison of this work with published data revealed that the absolute values of the interfacial reduction rates measured in this study are relatively low. This may be regarded as an indication that the overall reaction was slowed down by the presence of water vapor in the powder bed. In the light of this explanation, the impact of the different morphologies of the nc and the as-received powder on the overall reaction is rather negligible. ④ From the isothermal experiments, activation energies in the order of 50 kJ/mol were derived for this process. The results agree reasonable well with published data.

[Hydrogen reduction kinetics of alloy/composite nanopowders] iii) Case III : Optimization of hydrogen reduction of Fe2O3-NiO nanopowder It was found that the reduction process at all temperature conditions was completed within 140 min. Average grain sizes of Fe-Ni powder reduced at 450℃, 500℃ and 550℃ were 100 nm, 150 nm and 200 nm, respectively. From the hygrometry curves of the powders, the highest removal rate of water vapor appeared at 60 min, 51 min and 43 min, in which all powder samples were reduced over 95% and the average grain sizes were 70 nm, 90 nm and 120 nm, respectively. Therefore, it was expected that the removal of water vapor was delayed, because the trapped vapor remained in the reactor. Also, it should be noted that the delay of removal of water vapor during hydrogen reduction process is the most important factor affecting a subsequent thermodynamic and kinetic processes of Fe-Ni nanopowders.

iv) Case IV : Nonisothermal reduction kinetics of WO3-CuO nanopowder The effect of Cu content on hydrogen reduction behavior of ball-milled WO3-CuO nanocomposite powders was investigated. Hydrogen reduction behavior and reduction percent (a) of nanopowders were characterized by thermogravimetry (TG) and hygrometry measurements. Activation energy for hydrogen reduction of WO3 nanopowders with different Cu content was calculated at each heating rate and reduction percent (a). The activation energy for reduction of WO3 obtained in this study ranged from 129 to 139 kJ/mol, which was in accordance with the activation energy for WO3 powder reduction of conventional micron-sized powder. In the range of 0.1 ≤ a ≤ 0.4 and 0.7 ≤ a < 1.0, the activation energy for W-1wt.%Cu and W-5wt.%Cu nanopowders was 10% and 30% higher than those for reduction of pure W and WO2→W, respectively. Consequently, it was found that the reduction reaction of WO3 was retarded by the pre-reduced Cu particles.

Consequently, it is thought that the activation energy values calculated from hygrometry curves during reduction reaction are reasonable. Therefore, the hygrometry study during hydrogen reduction system can be applied to investigate the reduction kinetics as a new method. Also, it is expected that the real time control of water vapor removal during hydrogen reduction process by using in-situ hygrometry system enables us to optimize the reduction process for mass production of various metal nanopowders.