本文選題：鎂基儲(chǔ)氫合金 + 合金化��； 參考：《西北工業(yè)大學(xué)》2016年博士論文

【摘要】：鎂基合金以其儲(chǔ)氫密度大、重量輕、儲(chǔ)量豐富、可逆性好等優(yōu)點(diǎn)受到廣泛關(guān)注,被認(rèn)為是最有可能應(yīng)用于車載儲(chǔ)氫裝置的材料之一,但活化困難、放氫溫度高、吸/放氫動(dòng)力學(xué)緩慢成為其應(yīng)用的主要障礙。鎂基儲(chǔ)氫合金改性主要圍繞合金化、組織調(diào)控、表面改性等展開,方式單一,并未整體考慮吸/放氫反應(yīng)的完整過程�；谡w調(diào)控思路開發(fā)鎂基儲(chǔ)氫合金的手段很少涉及,且合金微結(jié)構(gòu)及性能之間的關(guān)系、催化改性合金活化機(jī)制及吸/放氫行為不夠明確。上述問題研究不僅能深入理解微結(jié)構(gòu)與儲(chǔ)氫性能之間的相關(guān)性,而且可以闡明催化劑在活化及后續(xù)吸/放氫中的作用,同時(shí)為綜合性能優(yōu)異鎂基合金體系的開發(fā)提供新思路。鑒于吸/放氫過程復(fù)雜性,本文基于整體調(diào)控思路,即高能球磨/熔體快淬優(yōu)化內(nèi)部組織基礎(chǔ)之上的表面催化改性獲得綜合性能優(yōu)良的儲(chǔ)氫合金�？紤]到氫能源汽車領(lǐng)域?qū)Υ笕萘績(jī)?chǔ)氫材料的迫切需求,基于微合金化思路,設(shè)計(jì)理論吸氫量高的富鎂合金Mg-10wt%Ni(Mg10Ni)進(jìn)行研究,并對(duì)傳統(tǒng)Mg_2Ni合金進(jìn)行挖潛。對(duì)基于整體調(diào)控思路制備的催化改性樣品,探討了微結(jié)構(gòu)與儲(chǔ)氫性能之間的構(gòu)效關(guān)系,揭示了催化改性Mg-Ni基合金的活化機(jī)制,研究了催化吸/放氫熱動(dòng)力學(xué)并探討了催化劑的去穩(wěn)定化作用。本文主要研究?jī)?nèi)容及取得的創(chuàng)新結(jié)果如下:采用SEM、TEM、EDS、XRD等手段研究了高能球磨及表面催化前后傳統(tǒng)Mg_2Ni合金微結(jié)構(gòu)及相組成,結(jié)果表明:熔鑄Mg_2Ni合金主要由包晶Mg_2Ni相構(gòu)成,還含有極少量殘余MgNi2相和Mg相。鑄態(tài)組織粗大,不利于快速完全吸/放氫,高能球磨可顯著細(xì)化合金組織,球磨不同時(shí)間可獲得不同組織狀態(tài)Mg_2Ni合金,球料比20:1時(shí),球磨38h后Mg_2Ni合金基本呈現(xiàn)非晶態(tài)。短時(shí)高能球磨引入MWCNTs,改善了表面特性而保留了組織調(diào)控后的非晶結(jié)構(gòu),實(shí)現(xiàn)了Mg_2Ni合金表面活性化與內(nèi)部精細(xì)化調(diào)控。非晶態(tài)Mg_2Ni合金的吸氫性能較優(yōu),30min內(nèi)可吸氫3.5wt%,且初始吸氫速率快,催化改性后最終吸氫量達(dá)到3.8wt%。微合金化Mg10Ni合金由先析出Mg枝晶相和層片共晶Mg-Mg_2Ni構(gòu)成,微觀組織粗大,比表面小,缺乏H擴(kuò)散通道;熔體快淬使基體Mg相及第二相Mg_2Ni納米化,合金組織有效細(xì)化,晶界/相界增多;Mg_2Ni納米化同時(shí)引入微孿晶,提高了富鎂合金的自催化效率。短時(shí)高能球磨使催化劑在合金顆粒表面均勻分布,為吸/放氫過程引入高活性質(zhì)點(diǎn),改善了表面特性。添加MWCNTs或TiF3可改善Mg10Ni合金活化特性;Mg10Ni-MWCNTs-Ti F3活化性能優(yōu)異,不存在孕育期,首次即可快速吸氫并達(dá)到理想吸氫量。XPS結(jié)果表明富鎂合金顆粒表面MgO和Mg(OH)2構(gòu)成的鈍化膜,降低了表面活性,阻礙了H_2吸附解離,造成合金活化困難。催化劑提高M(jìn)g10Ni活化性能不僅在于借助改善吸/放氫過程引起的晶格應(yīng)變促使表面已形成鈍化膜破碎并剝落,裸露出新鮮合金表面,還表現(xiàn)在阻礙表面鈍化膜形成。TiF3為H_2吸附解離及MgH_2異質(zhì)形核提供低能壘界面,加速吸氫反應(yīng);MWCNTs可促使H_2吸附及H快速擴(kuò)散;Mg10Ni-MWCNTs-TiF3活化過程中,MWCNTs對(duì)H的輔助擴(kuò)散與TiF3對(duì)H_2的吸附解離及對(duì)MgH_2的異質(zhì)形核相互協(xié)同作用而表現(xiàn)出優(yōu)異的活化性能。不同次數(shù)吸氫動(dòng)力學(xué)擬合指數(shù)m不同,尤其是催化改性前后Mg10Ni的首次吸氫動(dòng)力學(xué)擬合指數(shù)m差別較大,表明改性處理Mg10Ni合金首次吸氫過程的控速步驟不同,吸氫機(jī)制不同。在250℃,2.5MPa下,活化后Mg10Ni、Mg10Ni-MWCNTs、Mg10Ni-TiF3的吸氫量分別為5.39wt%、6.52wt%、5.06wt%;Mg10Ni-MWCNTs-TiF3,吸氫性能更優(yōu)異,前1min和5min可分別吸氫5.93wt%及5.99wt%;活化后Mg10Ni、Mg10Ni-MWCNTs、Mg10Ni-TiF3及Mg10Ni-MWCNTs-TiF3在300℃、2.5MPa下吸氫量分別達(dá)到6.01wt%、6.64wt%、5.89wt%和5.92wt%,300s內(nèi)吸氫量均超過5.0wt%。TEM及SEM相關(guān)結(jié)果及吸氫P-t曲線表明,TiF3促使H_2解離為活性H;TiF3自由表面及TiF3與基體界面為MgH_2異質(zhì)形核提供基底,導(dǎo)致吸氫初始階段合金表面形成大量氫化物核心,有限長(zhǎng)大后碰撞,形成MgH_2包覆Mg的核-殼結(jié)構(gòu),后續(xù)吸氫過程受控于H在MgH_2層中的緩慢擴(kuò)散,導(dǎo)致Mg10Ni-TiF3吸氫量較Mg10Ni降低;MWCNTs比表面活性高且具有管狀結(jié)構(gòu),有效促使H_2解離及H擴(kuò)散,可使H轉(zhuǎn)移到材料表面其它部位或次表面,降低了MgH_2初始形核率并加快H擴(kuò)散,使Mg10Ni在較低形核率下充分長(zhǎng)大,達(dá)到較高吸氫量;復(fù)合添加MWCNTs及TiF3后,TiF3解離的大量H可通過MWCNTs擴(kuò)散傳輸至合金表面其它部位或次表面,降低了MgH_2的異質(zhì)形核率,實(shí)現(xiàn)了飽和吸氫。溫度升高,吸/放氫熱力學(xué)P-C-T曲線之間的壓差逐漸減小,滯后效應(yīng)明顯降低,吸/放氫平臺(tái)更加平坦;催化改性Mg10Ni樣品滯后性存在顯著差異:Mg10Ni-MWCNTs滯后性較小,Mg10Ni-TiF3滯后性較大,Mg10Ni-MWCNTs-TiF3滯后性最小;通過Van’t Hoff方程擬合不同溫度條件下放氫熱力學(xué)P-C-T曲線,計(jì)算改性Mg10Ni體系氫化物生成焓和生成熵,從熱力學(xué)角度評(píng)價(jià)催化劑作用。相比Mg10Ni而言,添加MWCNTs和TiF3后,Mg10Ni體系氫化物生成焓從-77.36kJ/mol H_2分別降低至-73.75kJ/mol和-75.51kJ/mol,復(fù)合添加MWCNTs和TiF3后,氫化物形成焓降低至-73.37kJ/mol。富鎂合金體系氫化物形成焓降低表明催化劑添加可降低Mg-Ni體系氫化物穩(wěn)定性,改善合金放氫性能。
[Abstract]:Magnesium based alloys are widely concerned because of their high density of hydrogen storage, light weight, rich reserves and good reversibility. It is considered to be one of the most likely materials to be used in vehicle hydrogen storage devices, but it is difficult to activate, high hydrogen storage temperature and slow absorption / desorption kinetics as the main obstacle. The modification of magnesium based hydrogen storage alloys is mainly around alloying. The methods of developing magnesium based hydrogen storage alloys are rarely involved in the development of magnesium based hydrogen storage alloys, and the relationship between the microstructure and properties of the alloys, the activation mechanism of the modified alloys and the behavior of absorption / desorption are not clear enough. The above problems are not only studied. The relationship between microstructures and hydrogen storage properties can be understood in depth, and the role of catalysts in activation and subsequent desorption / desorption can be clarified, and a new idea is provided for the development of excellent comprehensive magnesium based alloy systems. In view of the complexity of the absorption / dehydrogenation process, this paper is based on the overall adjustment control idea, that is, the interior of high energy ball milling / melt quenching. On the basis of the surface catalytic modification, the hydrogen storage alloys with excellent comprehensive properties are obtained. Considering the urgent demand for large capacity hydrogen storage materials in the field of hydrogen energy vehicles, based on the micro alloying idea, the design of the magnesium rich alloy Mg-10wt%Ni (Mg10Ni) with high hydrogen absorption capacity is studied, and the traditional Mg_2Ni alloy is tapping the potential. The structure-activity relationship between the microstructure and the hydrogen storage properties was discussed. The activation mechanism of the catalytic modified Mg-Ni based alloy was revealed. The catalytic kinetics of catalytic desorption / desorption of hydrogen and the destabilization of the catalyst were studied. The main contents and the innovation results were as follows: using SEM, TEM, EDS, XRD The microstructure and phase composition of the traditional Mg_2Ni alloy before and after high energy ball milling and surface catalysis are studied. The results show that the melting and casting of Mg_2Ni alloy is mainly composed of peritectic Mg_2Ni phase, and contains a very small amount of residual MgNi2 and Mg phases. Mg_2Ni alloy with different tissue state can be obtained at the time. When the ball is compared to 20:1, the Mg_2Ni alloy is basically amorphous after ball milling 38h. Short time high energy ball milling is introduced to MWCNTs. The surface characteristics are improved and the amorphous structure controlled by the tissue is retained. The surface activity of the Mg_2Ni alloy and the regulation of the internal fine refinement are realized. The hydrogen absorption property of the amorphous Mg_2Ni alloy is achieved. It can be better, 30min can absorb hydrogen 3.5wt%, and the initial hydrogen absorption rate is fast. After catalytic modification, the final hydrogen absorption capacity of 3.8wt%. microalloyed Mg10Ni alloy is composed of Mg dendrite phase and lamellar eutectic Mg-Mg_2Ni. The microstructure is coarse, smaller than the surface and lack of H diffusion channel; melt rapid quenching makes the matrix Mg phase and the second phase Mg_2Ni nanoscale, alloy microstructure The increase of grain boundary / phase boundary and the introduction of micro twins in Mg_2Ni nanocrystalline can improve the autocatalytic efficiency of the magnesium rich alloy. Short time high-energy ball milling makes the catalyst distributed evenly on the surface of the alloy particles, introducing high active particles for the absorption / desorption process and improving the surface properties. The addition of MWCNTs or TiF3 can improve the activation properties of the Mg10Ni alloy; Mg10Ni-MW The activation performance of CNTs-Ti F3 is excellent, and there is no incubation period. It is the first time to quickly absorb hydrogen and reach the ideal hydrogen absorption capacity.XPS. The result shows that the passivation film composed of MgO and Mg (OH) 2 on the surface of the magnesium rich alloy particles reduces the surface activity, hinders the H_2 adsorption and dissociation and causes the alloy activation difficult. The catalyst improves the activation performance of Mg10Ni not only with the help of the improvement of the Mg10Ni. The lattice strain caused by the absorption / desorption process causes the surface to form the passivating film to break and exfoliate, expose the surface of the fresh alloy, and also show that the formation of.TiF3 is H_2 adsorption dissociation and the MgH_2 heterostructure nucleus provides the low energy barrier interface and accelerates the hydrogen absorption reaction; MWCNTs can promote the H_2 adsorption and the rapid diffusion of H; Mg10Ni-MWCNTs-TiF3 activation. During the process, the auxiliary diffusion of MWCNTs to H shows excellent activation performance with the adsorption and dissociation of TiF3 to H_2 and the synergistic effect of MgH_2 on the heterostructure of MgH_2. The kinetics fitting exponent m of different times of hydrogen absorption is different, especially for the first hydrogen absorption kinetic index m of Mg10Ni before and after the catalytic modification, which indicates that the modified Mg10Ni coincidence is modified. The mechanism of hydrogen absorption in the first hydrogen absorption process is different, and the mechanism of hydrogen absorption is different. At 250, 2.5MPa, the hydrogen absorption of Mg10Ni, Mg10Ni-MWCNTs, Mg10Ni-TiF3 after activation is 5.39wt%, 6.52wt%, 5.06wt%; Mg10Ni-MWCNTs-TiF3, and the hydrogen absorption performance is better, and the former 1min and 5min can absorb hydrogen 5.93wt% and 5.99wt% respectively. The hydrogen absorption of Mg10Ni-MWCNTs-TiF3 at 300 and 2.5MPa reached 6.01wt%, 6.64wt%, 5.89wt% and 5.92wt% respectively. The hydrogen absorption in 300s exceeded 5.0wt%.TEM and SEM related results and the hydrogen absorption P-t curve showed that TiF3 stimulated the dissociation of H_2 to be active, and the interface between the free surface and the matrix provided the substrate for the heterogeneous nucleation, leading to the initial stage of hydrogen absorption. A large number of hydride cores are formed on the surface of the alloy, and the nucleation and shell structure of MgH_2 coated Mg is formed after limited growth. The subsequent hydrogen absorption process is controlled by the slow diffusion of H in the MgH_2 layer, resulting in the reduction of Mg10Ni-TiF3 hydrogen absorption than Mg10Ni; MWCNTs is higher than the surface activity and has a tubular structure, which can effectively promote H_2 dissociation and H diffusion, and can transfer H to the material. On the other surface or subsurface, the initial nucleation rate of MgH_2 is reduced and the diffusion of H is accelerated so that the Mg10Ni is fully grown at a lower nucleation rate and achieves higher hydrogen absorption. After adding MWCNTs and TiF3, a large number of H can be transferred to the other parts or subsurfaces of the alloy surface through MWCNTs diffusion, and the heterogeneous nucleation rate of MgH_2 is reduced. The pressure difference between the thermodynamic P-C-T curves of absorption / desorption / desorption is gradually reduced, the lag effect is obviously reduced, the absorption / desorption platform is even more flat, and the hysteresis of the modified Mg10Ni samples has significant differences: Mg10Ni-MWCNTs hysteresis is smaller, Mg10Ni-TiF3 lagging is larger, Mg10Ni-MWCNTs-TiF3 lagging is the least; Van 't Ho is used. The FF equation fits the thermodynamic P-C-T curve of hydrogen release under different temperature conditions to calculate the hydride generation enthalpy and entropy of the modified Mg10Ni system, and evaluate the effect of the catalyst from the thermodynamic point of view. After adding MWCNTs and TiF3, the enthalpy of hydride generation in the Mg10Ni system decreases from -77.36kJ/mol H_2 to -73.75kJ/mol and -75.51kJ/mol, as compared with Mg10Ni. After adding MWCNTs and TiF3, the enthalpy of hydride formation decreased to the reduction of hydride formation enthalpy in the -73.37kJ/mol. rich magnesium alloy system, indicating that the addition of the catalyst can reduce the stability of the hydride in the Mg-Ni system and improve the hydrogen release properties of the alloy.

【學(xué)位授予單位】：西北工業(yè)大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類號(hào)】：TG146.22

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1 侯小江;Mg-Ni基合金的微結(jié)構(gòu)、吸放氫行為及其催化改性[D];西北工業(yè)大學(xué);2016年

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本文編號(hào)：1817331