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Among many metals the magnesium has excellent hydrogen-storage characteristics except that its hydriding and dehydriding rates are low Many works have been carried but in order to improve the reaction rates of magnesium with hydrogen. But their processes for the sample preparation were energy-consuming and complicated. In this work, to simplify the process of sample preparation, magnesium powder and cobalt powder were mixed in a 2 : 1 atomic ratio and pressed under 5 kbar to cylindrical pellet. The activation of the mixture was completed after about 15 hydriding-dehydriding cycles at about 700K and 25 barH₂The activated mixture absorbed hydrogen of about 2.8 weight percent at about 700K and 25 barH₂. The hydrogen storage capacity did not change until the 50th hydriding-dehydriding cycle. Equilibrium plateau pressures appeared at two different pressures, for example at 698K, 9 bar and about 14.5 bar. Under hydrogen pressures of relatively small deviations from the equilibrium plateau pressure, the hydriding reaction rates were controlled by the nucleation. After nucleation they were controlled by Knudsen flow and ordinary gaseous diffusion. Dehydriding reaction rates were controlled by the transformation of hydide into hydrogen and α-solid solution of Mg and Co.
Crofer22 APU specimens were prepared by grinding with grit 80 and 120 SiC grinding papers and were thermally cycled. The variation in oxidation behavior with thermal cycling was then investigated. Observation of microstructure, measurement of area specific resistance (ASR), analysis of the atomic percentages of the elements by EDX, and XRD analysis were performed. XRD patterns showed that the (Cr, Mn)3O4 spinel phase grew on the surface of the Crofer22 APU samples ground with grit 120. For the samples ground with grit 80, the ASR increased as the number of thermal cycles increased. Plots of ln (ASR/T) vs. 1/T for the samples ground with grit 80 after n = 4, 20 and 40 exhibited good linearity, and the apparent activation energies were between 63.7 kJ/mole and 76.3 kJ/mole.
Mg-x wt% Fe_2O_3-y wt% Ni samples were prepared by reactive mechanical grinding in a planetary ball mill, and their hydrogen-storage properties were investigated and compared. Activations of Mg-5Fe_2O_3-5Ni was completed after one hydriding (under 12 bar H_2) – dehydriding (in vacuum) cycle at 593 K. At n = 2,Mg-5Fe_2O_3-5Ni absorbed 3.43 wt% H for 5 min, 3.57 wt% H for 10 min, 3.76 wt% H for 20 min, and 3.98 wt%H for 60 min. Activated Mg-10Fe_2O_3 had the highest hydriding rate, absorbing 2.99 wt% H for 2.5 min,4.86 wt% H for 10 min, and 5.54 wt% H for 60 min at 593 K under 12 bar H_2. Activated Mg-10Fe_2O_3-5Ni had the highest dehydriding rate, desorbing 1.31 wt% H for 10 min, 2.91 wt% H for 30 min, and 3.83 wt%H for 60 min at 593 K under 1.0 bar H_2.
The LiNi1-yCoyO2 samples were synthesized at 800℃ and 850℃, by the solid-state reaction method, from the various starting materials LiOH, Li2CO3, NiO, NiCO3, Co3O4 and CoCO3., and their electrochemical properties are investigated. The LiNi1-yCoyO2 prepared from Li2CO3, NiO and Co3O4 exhibited the -NaFeO2 structure of the rhombohedral system(space group; Rm). As the Co content increased, the lattice parameters a and c decreased. The reason is that the radius of Co ion is smaller than that of Ni ion. The increase in c/a shows that two-dimensional structure develops better as the Co content increases. The LiNi0.7Co0.3O2[HOO(800,0.3)] synthesized at 800℃ from LiOH, NiO and Co3O4 exhibited the largest first discharge capacity 162 mAh/g. The size of particles increases roughly as the valve of y increases. The samples with the larger particles have the larger first discharge capacities. The cycling performances of the samples with the first discharge capacity larger than 150mAh/g were investigated. The LiNi0.9Co0.1O2[COO(850,0.1)] synthesized at 850℃ from Li2CO3, NiO and Co3O4 showed an excellent cycling performance. The sample with the larger first discharge capacity will be under the more severe lattice destruction, due to the expansion and contraction of the lattice during intercalation and deintercalation, than the sample with the smaller first discharge capacity. As the first discharge capacity increases, the capacity fading rate thus increases.
LiMn1.92Co0.08O4 and LiNi0.7Co0.3O2 synthesized by a simplified combustion method had good electrochemical properties. Mixtures LiMn1.92Co0.08O4 x wt.% LiNi0.7Co0.3O2 (x=9, 23, 33, 41 and 47) were prepared by milling for 30min and their electrochemical properties were investigated. The electrode with x=9 had a relatively large first discharge capacity (109.9 mAh/g at 0.1C) and good cycling performance. The decrease in the discharge capacity of the mixture electrodes with cycling is considered to result mainly from the degradation of LiNi0.7Co0.3O2, caused by coating of LiNi0.7Co0.3O2 with Mn dissolved from LiMn1.92Co0.08O4. 단순화한 연소법에 의해 합성한 LiMn1.92Co0.08O4와 LiNi0.7Co0.3O2의 혼합물의 전기화학적 성질을 알아보기 위하여, 30분동안 milling하여 LiMn1.92Co0.08O4 x wt.% LiNi0.7Co0.3O2 (x=9, 23, 33, 41 and 47) 조성의 혼합물을 제조하였다. x=9 조성의 전극이 비교적 큰 초기방전용량(109.9mAh/g at 0.1C)과 좋은 싸이클 성능을 가지고 있었다. 싸이클링에 따른 혼합물 전극의 방전용량 감소는 주로 LiNi0.7Co0.3O2의 퇴화에 기인한다고 생각된다. LiNi0.7Co0.3O2의 퇴화는 LiMn1.92Co0.08O4로부터 용해된 Mn이 LiNi0.7Co0.3O2입자를 둘러싸서(coating) 일어나는 것으로 판단된다.
LiCoyMn2-yO4 samples were synthesized by calcining a mixture of LiOH•H2O, MnO2 (CMD) and CoCO3 calcining at 400℃ for 10 h and then calcining twice at 750℃ for 24 h in air with intermediate grinding. All the synthesized samples exhibited XRD patterns for the cubic spinel phase with a space group Fd m. The electrochemical cells were charged and discharged for 30 cycles at a current density 600㎂/㎠ between 3.5 and 4.3 V. As the value of y increases, the size of particles becomes more homogeneous. The first discharge capacity decreases as the value of y increases, its value for y=0.00 being 92.8 mAh/g. The LiMn2O4 exhibits much better cycling performance than that reported earlier. The cycling performance increases as the value of y increases. The efficiency of discharge capacity is 98.9 % for y=0.30. The larger lattice parameter for the smaller value of y is related to the larger discharge capacity. The more quantity of the intercalated and the deintercalated Li in the sample with the larger discharge capacity brings about the larger capacity fading rate.
LiNi1-yGayO2 (y=0.005, 0.010, 0.025, 0.050 and 0.100) were synthesized by the solid-state reaction method after mechanical mixing, and their electrochemical properties were investigated. All the LiNi1-yGayO2 (y=0.005, 0.010, 0.025, 0.050 and 0.100) samples had the Rm structure. The sample with y=0.025 showed the largest first discharge capacity (131.4mAh/g) and good cycling performance [discharge capacity 117.5mAh/g (89.4% of the first discharge capacity) at the 20th cycle]. The first discharge capacity decreased as the value of y increased. The samples with y=0.010 and y=0.005 had small R-factor but their cycling performance was worse than that of the sample with y=0.025. All the LiNi1-yGayO2 samples had smaller discharge capacities than LiNiO2, but their cycling performances were better than that of LiNiO2. 기계적 혼합과 고상법에 의해 LiNi1-yGayO2 (y=0.005, 0.010, 0.025, 0.050 and 0.100)를 합성하고 전기화학적 특성을 조사하였다. LiNi1-yGayO2 (y=0.005, 0.010, 0.025, 0.050, and 0.100)의 모든 시료들은 Rm 구조의 상을 형성하였다. Ga가 y=0.025 치환된 경우 가장 높은 초기방전용량(131.4mAh/g)을 보여주었으며 20번째 싸이클에서 방전용량은 117.5mAh/g으로 초기방전용량의 약 89.4%이었다. Ga의 치환량이 증가함에 따라 초기방전용량의 감소가 뚜렷하였으며 Ga가 y=0.01과 y=0.005 치환된 경우 R-factor 값이 낮았지만 싸이클 성능이 Ga가 y=0.025 치환된 경우보다 좋지 않았다. Ga를 치환한 모든 조성에서 초기방전용량은 LiNiO2보다 줄어들었지만 싸이클 성능은 향상되었다.
기계적으로 합금처리한 Mg-18wt.%Ni 혼합물의 수소저장특성이 조사되었다. 1h, 3h, 그리고 6h 동안 기계적으로 합금처리한 혼합물들 중에서 6h동안 기계적으로 합금처리한 혼합물(MA 6h sample)이 가장 좋은 활성화, 수소화물 형성.분해 특성을 보인다. 수소화물 형성.분해 cycling을 시킴에 따라 $Mg_2$Ni상이 형성된다. MA 6h sample은 비교적 쉽게 활성화되며, 순수한 Mg나 Mg-10wt.%Ni 합금보다 수소화물 형성속도가 높으나, $Mg_2$Ni 합금보다는 수소화물 형성속도가 약간 낮다. MA 6h sample은 $Mg_2$Ni 합금에 비해 낮은 수소화물 분해속도를 보이지만, 순수한 Mg나 Mg-25wt.%Ni 합금보다는 높은 수소화물 분해속도를 보인다. MA 6h sample은 순수한 Mg나 다른 합금들보다 큰 수소저장용량을 가지고 있다. The hydrogen-storage properties of a mechanically-alloyed Mg-18wt.%Ni mixture were investigated. Among the mixtures mechanically alloyed for 1h, 3h, and 6h, the mixture mechanically alloyed for 6h(MA 6h sample) shows the best properties of activation, hydriding, and dehydriding. The $Mg_2Ni$ phase forms in the mechanically-alloyed Mg-18wt.%Ni mixture along with hydriding-dehydriding cycling. The MA 6h sample is relatively easily activated and has higher hydriding rate than the pure Mg, the Mg-10wt.%Ni alloy, and a little lower hydriding rate than the $Mg_2Ni$alloy. The MA 6h sample lower dehydriding rate than the $Mg_2$Ni alloy but higher dehydriding rate than the pure Mg and the Mg-25wt.%Ni alloy. The MA 6h sample has larger hydrogen-storage capacity than the pure Mg and the other alloys.
In our previous work, samples with a composition of 95 wt% Mg + 5 wt% CMC (Carboxymethylcellulose, Sodium Salt, [C6H7O2(OH)x(C2H2O3Na)y]n) (named Mg-5 wt%CMC) were prepared through hydride-forming milling. Mg-5 wt%CMC had a very high hydrogenation rate but a low dehydrogenation rate. Addition of Ni to Mg is known to increase the hydrogenation and dehydrogenation rates of Mg. We chose Ni as an additive to increase the dehydrogenation rate of Mg-5 wt%CMC. In this study, samples with a composition of 90 wt% Mg + 5 wt% CMC + 5 wt% Ni (named Mg-5 wt%CMC-5 wt%Ni) were made through hydride-forming milling, and the hydrogenation and dehydrogenation properties of the prepared samples were investigated. The activation of Mg-5 wt%CMC-5 wt%Ni was completed at the 3rd hydrogenation-dehydrogenation cycle (N=3). Mg-5 wt%CMC-5 wt%Ni had an effective hydrogen-storage capacity (the quantity of hydrogen stored for 60 min) of 5.83 wt% at 593 K in 12 bar hydrogen at N=3. Mg-5 wt%CMC-5 wt%Ni released hydrogen of 2.73 wt% for 10 min and 4.61 wt% for 60 min at 593 K in 1.0 bar hydrogen at N=3. Mg-5 wt%CMC-5 wt%Ni dehydrogenated at N=4 contained Mg and small amounts of MgO, β-MgH2, Mg2Ni, and Ni. Hydride-forming milling of Mg with CMC and Ni and Mg2Ni formed during hydrogenation-dehydrogenation cycling are believed to have increased the dehydrogenation rate of Mg-5 wt%CMC. As far as we know, this study is the first in which a polymer CMC and Ni were added to Mg by hydride-forming milling to improve the hydrogenation and dehydrogenation properties of Mg.