Research On Structures And Hydrogen Absorption/Desorption Performances Of V-Ti-Cr-Fe Alloys

As an important kind of hydrogen storage materials, vanadium-based alloysarose popular interests due to their performances of absorbing/desorbing hydrogenfast at room temperature and high capacities beyond 3.8wtï¼…H theoretically.V-Ti-based alloys prepared by pure vanadium metal, such as V-Ti-Cr alloys, showhigh desorption capacities about 2.5wtï¼…H, but are high costly and poor activated.Some researchers have tried to produce low-cost V-based alloys by FeV80 masteralloy, but none with high desorption capacity was obtained. Due to these problems,based on literatures, many efforts have been made in this thesis by compositionmodification, XRD and SEM/EDS analysis, PCT measurement, and mathematicscalculation. As a result, a novel V-Ti-Cr-Fe alloy system was built first to reveal thefactors influencing dominantly the hydrogen absorption/desorption properties ofV-Ti-Cr-Fe BCC alloys and develop a serial of Fe-containing BCC alloys with V/Feratio of 5 with high absorption/desorption capacities. The effects of impurities inFeV80 master alloys, such as Al and Si, on the microstructure and hydrogenabsorption/desorption properties were researched. The possibility of preparinghigh-capacity BCC alloys by FeV80 master alloy was investigated. The activationproperties of BCC alloys were improved remarkably by small addition of rare-earth metals, and finally low-cost, high-activity and high-capacity V-based alloys weredeveloped. The main achievements were made as follows:The hydrogen absorption/desorption properties of V-Ti-Cr-Fe BCC alloys weredependent mainly on lattice parameter (a) and electron concentration (C_e). Whena＜0.3030nm, high absorption capacities are difficult to be obtained, which increasewith the increase of a. When a≥0.3030nm, the alloys show high absorptioncapacities, which tend to be stable with the further increase of a. The hydrogendesorption capacities increase first and then decrease with the increase of a, showinghigh values in the lattice parameter range of 0.3030／0.3050nm. According to therelationship between lattice parameters and alloy compositionsand that betweenlattice parameters and hydrogen desorption capacities, the maximum desorptioncapacity was obtained at 0.3036nm of a. Hydrogen absorption capacity is impossibleto be high when C_e＞5.25, and it decreases notably when C_e increases further. Highdesorption capacities can be obtained only when 5.13＜C_e＜5.25.Based on the relationship between alloy compositions and lattice parameters,the V-Ti-Cr-Fe alloy system was built in the range of 15～60atï¼…V when a wasassigned to be around 0.3036nm and V/Fe ratio was set to be 5. All alloys in thissystem show single BCC structures with lattice paramerters in the range of0.3040nm～0.3053nm and electron concentration in the range of 5.14～5.23, andespecially the possibility to be prepared by low-cost FeV80 master alloy. Theobtained alloys with 20at％～55atï¼…V show high absorption/desorption capacities,which absorb＞3.55wtï¼…H and desorb＞2.03wtï¼…H at 298K. It is difficult to obtainhigh capacities when V＜20atï¼…or V＞55atï¼…. Notably, when 30at％＜V＜45atï¼…, theobtained alloys show familiar microstructures, crystallographic structures andhydrogen absorption/desorption properties, whose lattice parameters are all around0.3042nm, absorption capacities＞3.70wtï¼…and desorption capacities≥2.20wtï¼….Among these alloys, V₄₀Ti_28.4Cr_23.6Fe₈ shows the maximum absorption/desorptioncapacity, absorbing 3.80wtï¼…H and desorbing 2.30wtï¼…H at 298K, which are themaximum in V-Ti-Cr-Fe alloy system.Concerning the impurity of Al in FeV80 master alloy, the effects of 0～5atï¼…Al addition on V₃₀Ti₃₅Cr₂₅Fe₁₀ BCC alloy were researched. After the addition of Al, thealloy keeps still single-phase with BCC structure, and its microstructure doesn'tchange evidently. However, the lattice parameter increases, hydrogenabsorption/desorption capacity decreases, and plateau pressure rises. With theincrease of Al content, hydrogen absorption/desorption capacity decreases, andplateau pressure rises further. When Al≤1.5atï¼…, the hydrogen absorption/desorptioncapacity has no evident reduction, the former exceeds 3.44wtï¼…and the latter1.84wtï¼….To concern the impurity of Si in FeV80 master alloy, the effects of 0～2.5atï¼…Siaddition in V₃₀Ti₃₅Cr₂₅Fe₁₀ BCC alloy were investigated. Si induces the formation ofC14 Laves phase in V₃₀Ti₃₅Cr₂₅Fe₁₀ BCC alloy. When Si increases, the content ofC14 Laves phase increases, which leads to the decrease of both BCC phase and itslattice parameter. Laves phase, in which Si is concentrated, mainly grows along thegrain boundaries. The appearance of Laves phase improves the activation propertiesof V₃₀Ti₃₅Cr₂₅Fe₁₀ BCC alloy significantly, which contributes to the fast reactionbetween Si-containing alloy and hydrogen at 298K after a short period of incubation.The incubation is shortened When Si content increases. However, Si addition leads tothe decrease of absorption/desorption capacities, increase of plateau pressure andslope of the plateau region. When Si≤1.0atï¼…, there is a little decrease in hydrogenabsorption capacity, which reduces greatly when Si＞1.0atï¼…. As for desorptioncapacity, it doesn't drop remarkably when Si≤0.63atï¼…, while it descends noticeablywhen Si content exceeds 0.63atï¼….According to the above, (30～70)V_x-Ti-Cr-Fe(Ti:Cr:Fe=35:25:10) alloys with1.0atï¼…Si addition were studied. The results indicate that increasing V contentcontributes to the increase of BCC phase and then leads to the increase of hydrogenabsorption/desorption capacities. For the alloys with V≥42atï¼…, 1.0atï¼…Si additionimproves the activation performance and plateau pressure evidently but influenceslittle on the hydrogen absorption/desorption capacities. V₃₀Ti₃₅Cr₂₅Fe_10-Al_1.25Si_x alloys consist of both BCC phase and C14 Laves phase.With the increase of Si content, the amount of Laves phase increases, the hydrogenabsorption/desorption capacities decrease and the plateau pressures rise. Evidently,the effects of Si on V₃₀Ti₃₅Cr₂₅Fe_10^-Al_1.25 alloy are similar to those onV₃₀Ti₃₅Cr₂₅Fe₁₀ alloy. Thus, the effects of Al and Si are independent when Al and Siwere added into BCC alloy together. It was found that V₃₀Ti₃₅Cr₂₅Fe_10^-Al_1.25Si_xalloys with Si≤0.3atï¼…show higher absorption/desorption capacities comparable toV₃₀Ti₃₅Cr₂₅Fe₁₀ alloy, which indicates the possibility of producing V-based alloyswith high capacities by FeV80 master alloy.The function of rare-earth metals was tested. After 0.5～3atï¼…Ce addition, theactivation performance of V₅₅Ti_22.5Cr_16.1Fe_6.4 BCC alloy was improved remarkably,which absorbs hydrogen quickly at room temperature without any activationtreatment. It was found that Ce disperses in the alloy rather than dissolves into BCCphase. The addition of Ce hasn't evident effect on the PCT properties of the matrixalloy. La, Pr and Nd have similar function to Ce. It is the high activity of rare-earthmetals that helps the alloy to be easily activated and absorb hydrogen quickly atroom temperature.The as-cast V₃₀Ti₃₅Cr₂₅Fe₁₀ alloy prepared by FeV80 master, alloy and a smallquantity of rare-earth metals was investigated. It's composed of BCC phase, smallamount of C14 Laves phase and rare-earth metal one. BCC phase exists mainly asdendrite, and C14 Laves phase grows mainly between the branches, while rare-earthmetal phase disperses randomly. The alloy shows good activation performance at298K, which can react with hydrogen after an incubation of 30s and nearly reach thesaturation state in the subsequent 5min. It absorbs 3.60wtï¼…H and desorbs 1.88wtï¼…Hat 298K, showing comparative properties to V₃₀Ti₃₅Cr₂₅Fe₁₀ produced by pure Vmetal. The formation enthalpy was calculated to be -47.9 kJ/mol·H₂.To improve the hydrogen desorption properties of V₃₀Ti₃₅Cr₂₅Fe₁₀ alloyprepared by FeV80 master alloy and a small quantity of rare-earth metal,heat-treatments under high temperatures (1373K～1673K) were made, and the effectsof both temperatures and durations were investigated. When the tempearture＜ 1573K, Laves phase grew up between the branches of the dendrites, leading to theremarkable decrease of absorption/desorption capacity. When annealed at 1573K or1673K, the content of C14 Laves phase decreased sharply, and the dendritesdisappeared and were converted to equiaxed grains. The grains grew up andsubcrystals appeared in the grains besides very small amount of C14 Laves phaseand rare-earth metal phase. The absorption/desorption capacities and plateau flatnessincrease evidently due to the decrease of C14 Laves phase and the improvement ofhomogeneity. After being annealed at 1673K, the alloy shows the highestabsorption/desorption capacity about 3.66wtï¼…H/2.05wtï¼…H, respectively.Heat-treatment duration has no evident influence on alloy when annealingtemperature was fixed to 1673K, and similar microstructures, crystallographicstructures and absorption/des0rption properties were obatined for different annealingdurations from 10min to 120min. A higher desorption capacity of 2.05wtï¼…H wasobtained for annealing time of 30min and 120min.The (20～55)V-Ti-Cr-Fe alloys were researched, which were prepared by FeV80master alloy and a small quantity of rare-earth metal and then heat-treated at 1673Kfor 30min. For 20atï¼…V alloy, it consists of BCC phase and a large quantity ofC15Laves phase, so that it only absorbs 2.50wtï¼…H and desorbs 1.05wtï¼…H at 298K.When 25at％≦V≦40atï¼…, the alloys show nearly single BCC phases, whichabsorb＞3.65wtï¼…H and desorb＞2.23wtï¼…H at 298K. Among these alloys,V₃₀Ti_32.4Cr_31.6Fe₆ alloy shows the highest desorption capacity of 2.35wtï¼…H at 298Kand 2.56wtï¼…at 373K, which is also the highest in Fe-containing alloys as reportedso far and comparative in properties to the V-Ti-Cr alloys developed successfully byresearchers in Japan. When V content exceeds 40atï¼…, the alloys prepared by FeV80master alloy show capacities lower than 3.50wtï¼…and desorption capacities lowerthan 2.0wtï¼…at 298K though they are nearly BCC single phases.