钒基合金/碳复合材料吸放氢过程的晶体缺陷与相变演化

The theoretical hydrogen storage capacity of vanadium is 3.8wt%. Unfortunately, its reversible capacity is approximately 60% of that due to low-pressure plateau during hydrogenation/dehydrogenation process. It is demonstrated that V-based hydrogen storage alloy with ultramicro scale(1-10mm) and less crystal defects can exhibit single high-pressure hydrogenation up to 3.3wt%, attributing to the vanishing of low-pressure plateau (i.e.β phase) and the appearance of high-pressure plateau (i.e. g phase precipitating directly from α hydrogen containing solution phase) induced by defects. The applicants predict that the crystal defects accumulated with g phase increase lead to the precipitation of β phase during dehydrogenation, which results in an equivalent effective hydrogen storage capacity of the ultramicro alloy with that of the normal alloys. However, the research indicates that the ultramicro scale cannot inhibit the follow-up defect accumulation during hydrogen uptake-release process and thus cannot improve the dehydrogenation performance as well. In this proposal, we plan to make V-based hydrogen storage alloys in sub-micron to nanometer in order to prohibit the generation of internal stress and crystal defects during hydrogenation/dehydrogenation uttermostly. The generation of internal stress, dislocation/vacancies and the variation of defect densities of V-based hydrogen storage alloys in sub-micro to nanometer after different hydrogenation/dehydrogenation cycles and under various ab/desorption amount will be studied. The principle of phase transformation and hydrogenation/dehydrogenation characteristics corresponding to defects will be investigated. In order to prevent the highly active sub-micro to nanoparticles from agglomeration and surface contamination, carbon and graphene wrapping will be introduced. Corresponding results are expected to provide theoretical basis for the manufacturing of V-based hydrogen storage alloy/carbon composite materials with high effective hydrogen storage capacity and excellent cycling stability.

金属钒理论贮氢量为3.8wt%，遗憾的是其存在低压吸放氢平台的问题，可用放氢量仅60%。已有研究表明，超微尺度（1-10mm）且晶体缺陷少的钒基贮氢合金可实现超过3.3wt%的单一高平台吸氢，本质是缺陷诱导的低平台β相消失，高平台γ相直接从α氢固溶体析出。申请人推测随γ相增多而积累的晶体缺陷造成放氢过程β相析出，导致超微贮氢合金有效放氢量与常规合金相当，但该研究暗示超微尺度不能阻止后续吸放氢过程的缺陷堆积而改善放氢性能。本申请提出将钒基贮氢合金尺度向亚微米-纳米方向延伸，最大限度阻止吸放氢过程内应力及晶体缺陷产生。将研究不同循环次数及不同吸放氢量时，亚微米到纳米尺度的钒基贮氢合金内应力、位错与空位的形成与缺陷密度变化，研究与缺陷对应的相变和吸放氢规律。为防止高活性的亚微米和纳米颗粒的团聚和表面污染，引入碳及石墨烯包覆。最终为制备放氢量高、循环稳定性好的钒基贮氢合金/碳复合材料提供理论依据。

钒基储氢合金中体相和表面缺陷与一氢化物和二氢化物的相变和合金的吸放氢性能三者之间强相关。本课题研究了钒基储氢合金/碳复合材料精细化技术，系统研究了缺陷、相变和性能之间的关系。.湿磨行星球磨法制备的(VFe)48(TiCr)49Mn3合金粉的细化效果比高能球磨好，等离子法的严重氧化。但合金颗粒细化过程造成微观应变累积，恶化了吸放氢性能。.氟化处理(VFe)48(TiCr)49Mn3合金粉有效去除氧化层，提高了放氢量。但形成MnF2和CrF3，降低了吸放氢动力学性能，循环性能变差。酸处理对合金粉表面污染加剧，吸放氢性能变差。.行星球磨法成功制备了不同尺度（微米~纳米）V60Ti25Cr3Fe12合金粉及其碳复合材料。重点研究了不同球磨时间V60Ti25Cr3Fe12合金粉的微结构与吸放氢性能及相转变的关系。球磨时间从0.5h到3h，合金粉平均尺寸从6.20μm降到1.57μm，微观应变从0.640%增至0.675%，位错密度从2.38*1014m-2增至2.55*1014m-2。缺陷抑制了V2H与VH2之间的相转变，使合金的放氢量从1.48wt%减少到1.21wt%。颗粒细化有利于吸氢动力学性能，但缺陷无法完全消除，使循环性能恶化。高温热处理有效的控制了合金缺陷。提高热处理温度减少了球磨引入的微观应变、非晶化、晶格畸变和位错，使放氢量从400℃热处理后的0.98wt%增至1000℃的1.67wt%。但即使1000℃热处理后依然有部分微观应变和位错，使放氢量小于预期值。.球磨法制备(VFe)48(TiCr)49Mn3/炭黑复合材料，5次循环后加4wt%炭黑比未加碳的样品的放氢容量衰减率、微观应变和位错密度均有减小。长时间球磨成功制备了纳米级V60Ti25Cr3Fe12-乙炔黑/石墨烯复合材料。球磨12h后合金粉的平均粒径在100nm左右，但球磨时间过长使合金结构遭到严重破坏，400℃热处理后依然存在大量缺陷，使放氢量分别为0.68wt%和0.35wt%。V60Ti25Cr3Fe12/石墨烯的放氢量更少是因为石墨烯分散剂对储氢合金有毒化作用。.球磨法可细化钒基储氢合金至纳米级，但同时产生内应力、晶格畸变、位错、非晶化和表面钝化问题，即使高温热处理仍难以消除。采用超薄合金薄膜等方法实现合金超小尺度化，可望获得结构完整性高、性能优异的钒基储氢合金。