氮、氧杂质对<bold>V40Ti26Cr26Fe8 </bold>合金化学成分、微观组织和贮氢性能的影响

朱鹏飞; 王倩; 吴朝玲; PengFei Zhu; Qian Wang; ChaoLing Wu

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Effect of Nitrogen and Oxygen Impurities on Chemical Composition, Microstructure, and Hydrogen Storage Performance of V₄₀Ti₂₆Cr₂₆Fe₈ Alloy PDF

- ORCID：
Zhu Pengfei ¹
✉
- ORCID：
Wang Qian ¹
- ORCID：
Wu Chaoling ^1,3
✉
- ORCID：
Chen Yungui ^2,3
- ORCID：
Yan Yigang ^2,3
- ORCID：
Wang Yao ^2,3

1. College of Materials Science and Engineering, Sichuan University, Chengdu 610064, China； 2. Institute of New-Energy and Low-Carbon Technology, Sichuan University, Chengdu 610065, China； 3. Engineering Research Center of Alternative Energy Materials & Devices, Ministry of Education, Chengdu 610064, China

Updated：2022-05-26

Abstract

The synergetic effects of nitrogen and oxygen impurities on the micro-area chemical composition, microstructure, and hydrogen storage performance of V-Ti-Cr-Fe alloys were investigated through X-ray diffraction, scanning electron microscopy, optical microscopy, and pressure-composition-temperature (PCT) analyses. Results show that the hydrogen storage capacity of V₄₀Ti₂₆Cr₂₆Fe₈ alloy is obviously decreased with increasing the nitrogen and oxygen impurities. The mechanism for the decrease is as follows: the oxygen dissolved in V-based alloys suppresses the formation of the dihydride phase, resulting in the decrease in hydrogen capacity (primary effect); while the nitrogen is bound with titanium forming a new nitrogen-titanium-enriched phase, which results in composition change of the main phase and decreases the lattice parameter (secondary effect).

Keywords

V-Ti-Cr-Fe alloy; hydrogen storage performance; nitrogen and oxygen impurities; lattice parameter

Science Press

Hydrogen is a suitable and environmental-friendly energy storage medium because of its easy availability and eco-friendliness of non-polluting products after combustion^[

1]. The efficient storage of hydrogen is one of the key technical problems in the hydrogen energy field. Compared with the high-pressure gaseous hydrogen storage and cryogenic liquid hydrogen storage, the solid hydrogen storage method has the merits of high volumetric hydrogen storage density and high safety^{[Reference 2

Baidu Scholar}2].

The vanadium-based alloy has a high hydrogen storage capacity of around 3.8wt% in theory and excellent hydrogenation-dehydrogenation kinetics under moderate temperature and pressure conditions^[

3]. Many studies have been conducted to improve the hydrogen storage performance of V-based materials. The application of ferro-vanadium alloy greatly reduces the production cost of the V-based alloy and the iron has few adverse effects on hydrogen storage capacity, thereby building a foundation for the industrialized production of V-based hydrogen storage materials^{[Reference 4

Baidu Scholar}4,5]. The heat treatment can eliminate some defects in the as-cast alloys, result in lower thermal stability of the hydride, and improve the activation performance and kinetic properties^{[Reference 6

Baidu Scholar}6]. Luo et al^{[Reference 7

Baidu Scholar}7] showed that the smaller the particle size of V-based alloy in the range of 38~250 μm, the lower the micro-strain in the lattice owing to the easier micro-stress release, and thus the better the hydrogen absorption-desorption cycle performance. The addition of alloying element can modify the interactions between the specimen and hydrogen, which drastically improves the hydrogen absorption characteristics^{[Reference 8-17}8-17]. Yan et al^{[Reference 18

Baidu Scholar}18,19] found that Al and Si impurities in raw materials have great influences on hydrogen storage properties of V-based alloy. The effects of oxygen on microstructure and hydrogen storage performance for V-based alloy were reported in Ref.[20-22]. However, most reports only focus on the oxygen impurity, ignoring the possible changes in the microstructure and chemical composition in the micro-area of alloys when other impurities are introduced in process.

The effects of nitrogen impurities on the microstructure and properties of V-based hydrogen storage alloys are rarely reported. The nitrogen and oxygen impurities are possibly introduced in the high-temperature process of arc melting and/or heat treatment. In this research, the synergistic effects of nitrogen and oxygen impurities on the chemical composition, microstructure, and hydrogen storage performance of V-based alloys were investigated.

1 Experiment

Three ingots of V₄₀Ti₂₆Cr₂₆Fe₈ alloy were named as specimen A, specimen B, and specimen C and were prepared by arc melting under argon-oxygen-nitrogen mixtures in different proportions in the ferro-vanadium materials. The content of Ti and Cr purities was 99.8wt% and 99.8wt%, respectively. The nitrogen and oxygen contents of the three alloys were analyzed by the inert gas fusion-infrared radiation (IR) method (TCH-600, LECO). The difference in oxygen and nitrogen contents is listed in Table 1.

Table 1 Nitrogen and oxygen contents of specimen A, specimen B, and specimen C (wt%)

Specimen	N	O
A	0.024	0.045
B	0.202	0.078
C	0.264	0.170

The specimen A had a little nitrogen and oxygen, specimen B contained the moderate content of nitrogen and oxygen, and specimen C was rich in nitrogen and oxygen. Each alloy was turned over and remelted at least four times to ensure the homogeneity of chemical composition, and then annealed at 1623 K for 30 min under vacuum condition in ZM-16 vacuum molybdenum wire furnace followed by furnace cooling to room temperature.

The crystal structures of the alloys were analyzed by X-ray diffractometer (XRD, DX-2700B) with an operating voltage of 40 kV and current of 30 mA. The structure refinement was conducted by marching step scanning under the conditions of scanning speed of 0.05°/s, sampling time of 2 s, and scanning angle of 30°~90°.The structure and morphology were ob-served using an optical microscope (OM, JX-2000B). The alloys were ground successively with various sandpaper and polished with metallographic polishing paste. The well-polished surfaces were further corroded by a corrosive agent of 10vol% HF+20vol% HNO₃+70vol% H₂O, then rinsed with deionized water, cleaned with alcohol, and finally blow-dried. The alloy microtopography and micro-area chemical composition were analyzed by scanning electron microscope (SEM, U-8200) equipped with energy dispersive spectrometer (EDS, Oxford X-Max). Furthermore, the composition of main and secondary phases was determined by EDS.

The Sieverts-type device was applied to study the hydrogen absorption-desorption properties of the alloys at 298 K under the cut-off pressure of 0.001 MPa. Firstly, all alloys were broken into small pieces of 1~4 mm in size and then put into a stainless-steel reactor for initial hydrogen absorption kinetics and pressure-composition-temperature (PCT) tests. The rea-ctor containing alloy pieces was vacuumized at 673 K for 30 min, and then slowly cooled down to room temperature. The hydrogen (99.999wt%) with 4 MPa pressure was introduced into the reactor and the molar amount of hydrogen partici-pating in the reaction was accurately calculated through the data from pressure gauge.

2 Results and Discussion

2.1 Microstructure and micro-area chemical composition analysis

The microstructures of the V₄₀Ti₂₆Cr₂₆Fe₈ alloys with different contents of N and O impurities are shown in Fig.1. The three alloys are all composed of two phases: the gray main phase and the black secondary phase. In addition, the amount of the secondary phase is increased with increasing the N and O impurities. EDS results of different phases in specimens are listed in Table 2 to demonstrate the detailed composition. It is clear that Ti content in the secondary phase is significantly higher than in the main phase, suggesting that a titanium-rich phase is generated. The nitrogen is almost completely enriched in the secondary phase, and its content is significantly increased with increasing the N and O impurities. Meanwhile, the oxygen is dispersed throughout specimens without significant content difference in the main and secondary phases, whereas its content is increased with increasing the N and O impurities, which is consistent with results in Table 1.

Fig.1 SEM microstructures of V₄₀Ti₂₆Cr₂₆Fe₈ alloys of specimen A (a), specimen B (b), and specimen C (c)

Table 2 EDS results of chemical composition of different phases in V₄₀Ti₂₆Cr₂₆Fe₈ alloy specimens (wt%)

Specimen	Phase	V	Ti	Cr	Fe	N	O
A	Main phase	40.08	25.70	24.78	7.37	0	2.05
A	Secondary phase	24.27	53.83	13.28	3.84	1.57	3.21
B	Main phase	38.90	24.87	24.97	7.40	0	3.87
B	Secondary phase	18.45	55.26	12.33	2.66	6.67	4.81
C	Main phase	38.01	23.89	24.76	7.37	0	6.13
C	Secondary phase	14.21	60.17	8.28	2.33	8.51	6.32

Fig.2 shows the morphologies of the enriched secondary phase in specimen C. The nitrogen is enriched in the secondary phase, while oxygen is dissolved throughout in the V-based alloys without prominent compositional fluctuation.

Fig.2 SEM image of enriched secondary phase in specimen C (a); SEM image of magnified circle area in Fig.2a (b) and corresponding element distributions of V, Ti, Cr, Fe, N, O for secondary phase (c)

2.2 Metallographic structure analysis

The observed grain morphology is simplified as a cuboid with specific length, width, and height, as shown in Fig.3. The metallographic microstructures in longitudinal section and cross section of specimen A, specimen B, and specimen C are shown in Fig.4. The specimen A is composed of columnar grains, specimen C is composed of equiaxed grains, and specimen B has the mixture structure of columnar grains and equiaxed grains. The average grain sizes of hypothetical grain in different specimens are shown in Table 3. The width and height are almost the same while the length (L) is quite different among different specimens with L_A:L_B:L_C=8:4:1 (L_A, L_B, and L_C represent the grain length on the longitudinal section of specimen A, specimen B and specimen C, respectively). The crystal growth in specimen A has obvious directionality, while that in specimen C has no directionality, and that in specimen B shows a relatively weak directionality. Therefore, the impurities play a role in grain size regulation: the less the nitrogen and oxygen in alloy, the larger the grains.

Fig.3 Schematic diagram of grain morphologies with specific length, width, and height of specimen A, B, and C

Fig.4 OM images of longitudinal sections (a~c) and cross sections (d~f) of microstructures in specimen A (a, d), specimen B (b, e) and specimen C (c, f)

Table 3 Average grain sizes of hypothetical grains in specimen A, B, and C (μm)

Specimen	Length	Width	Height
A	900	141	101
B	469	142	134
C	110	142	115

From the perspective of thermodynamics, the homogeneous nucleation is spontaneous due to the supercooling, even in the liquid. In fact, the metal inevitably contains some refractory impurities which are distributed in the liquid in the melting process. The impurities provide a favorable surface for the generation of crystal nucleuses in the liquid and reduce the interface energy. This process is heterogeneous nucleation, which can be expressed by Eq.(1), as follows:

Δ W_{h e}^{*} = \frac{16}{3} \frac{π σ_{L c}^{3} T_{m}^{2}}{3 L^{2} {(Δ T)}^{2}} [\frac{2 - 3 c o s θ + c o s θ^{3}}{4}] = Δ W_{h o}^{*} f (θ)

(1)

where $Δ W_{h e}^{*}$ is heterogeneous nucleation work, $Δ W_{h o}^{*}$ is homogeneous nucleation work, σ_Lc is the unit interface free energy between the liquid phase and the crystal nucleus, T_m is the melting point of the metal, L is the absorbed latent heat of fusion during the transition from solid phase to liquid phase, ΔT is the undercooling degree, and f(θ) is a function of θ angle with 0°＜θ＜180°. As 0°＜θ＜180° and -1＜cosθ＜1, f(θ) changes within the range of -1＜f(θ)＜1, as shown in Eq.(2):

f (θ) = \frac{2 - 3 c o s θ + c o s θ^{3}}{4}

(2)

So the heterogeneous nucleation work is less than the homogeneous nucleation work. The refractory impurities distributed in the liquid create favorable conditions for the generation of crystal nucleuses during the process of heterogeneous nucleation on the surface of these impurities, as shown in Fig.5. The nitrogen impurity introduced in processing is bound with titanium to form nitrogen-titanium-enriched secondary phase, which has a much higher melting point than other substances in the melting process. During the cooling and crystallization, the secondary phase acts as a heterogeneous crystal nucleus to promote the V-based alloy nucleation. The nitrogen-titanium-enriched secondary phase is distributed uniformly in the V-based alloy liquid as a grain refiner, as shown in Fig.6. Thus, the more the impurities in the alloy solution, the more the favorable sites for nucleation and the smaller the crystal grains.

Fig.5 Schematic diagram of heterogeneous nucleation in V-based alloy

Fig.6 Schematic diagram of nitrogen-titanium-enriched phase as grain refiner in V-based alloy

2.3 Crystal structure of alloy before and after hydrogena-tion

XRD analysis was applied to investigate the evolution of crystal structure during the whole process of hydrogen storage and release. The XRD patterns of the three alloys before hydrogen absorption are shown in Fig.7a. Although the metallographic structures of different specimens are quite different, XRD patterns of all the specimens are similar, indicating that they are all composed of the single V-based body-centered cubic (bcc) phase, namely the hydrogen absorbing phase. The content of the secondary phase in nitrogen and titanium of the three specimens is too insufficient to be detected by XRD. Therefore, the Rietveld refinement method is used, as shown in Fig.8. The detailed lattice parameters of the main bcc phase of specimen A, B, and C are 0.3036, 0.3023, and 0.3015 nm, respectively, as shown in Table 4. The lattice parameter is decreasesd with increasing the impurity content, because the content of Ti which has a larger atomic size than V, Fe, and Cr is decreased in the main phase due to the formation of nitrogen-titanium-enriched phase, which is consistent with the results in Ref.[

23].

Table 4 Lattice parameters of main bcc phase in V₄₀Ti₂₆Cr₂₆Fe₈ alloy specimens before hydrogenation (nm)

Specimen	Lattice parameter
A	0.3036
B	0.3023
C	0.3015

The hydrogen absorption kinetics experiments were conducted firstly, and then XRD analysis was performed on the powders after hydrogenation, as shown in Fig.7b. The crystal structures of the three specimens after hydrogen absorption are quite different. Specimen A and B after hydrogenation are composed of two phases: VH₂ and V₄H_2.88. The diffraction peak intensity of VH₂ phase of specimen B is weaker than that of specimen A, which demonstrates that less VH₂ phase is generated in specimen B. Specimen C mainly consists of V₄H_2.88 phase. The contents of VH₂ phase in specimen A, B, and C calculated by Rietveld refinement method are 33.3wt%, 13.4wt%, and 0.6wt%, respectively, as shown in Table 5. The content of VH₂ phase is decreased with increasing the impurity content. It is reported that the oxygen dissolved in V-based alloy suppresses the formation of the dihydride^[

22]. Thus, it can be deduced that oxygen is one of the critical factors to reduce the hydrogen storage capacity of the alloy.

Table 5 Hydride phase contents of V₄₀Ti₂₆Cr₂₆Fe₈ specimens after hydrogenation (wt%)

Specimen	VH₂	V₄H_2.88
A	33.3	66.7
B	13.4	86.6
C	0.6	99.4

All dehydrogenated specimens only contain the single V₄H_2.88 phase, as shown in Fig.7c. It is concluded that the VH₂ phase disappears during the dehydrogenation process, but V₄H_2.88 remains. This is because the dihydride phase is un-stable and easily decomposes under the moderate temperature and pressure, while the V₄H_2.88 phase is thermally stable, which requires much higher temperature for dehydrogenation^[

4].

2.4 Performance of hydrogen absorption and desorption

The hydrogen absorption kinetic curve is usually used to study the hydrogen absorption ability. As shown in Fig.9, at 298 K, the specimen A has an excellent hydrogen absorption kinetic with a maximum hydrogen absorption capacity of 3.94wt% in 40 min. The rapid adsorption occurs in the first 5 min, and 79% of the maximum amount of hydrogen is absorbed. However, the specimen B only absorbs maximally 2.03wt% hydrogen, and specimen C has the lowest hydrogen absorption capacity of only 1.58wt% in the same duration. The PCT curves of the hydrogen release are shown in Fig.10. The specimen A has the largest hydrogen desorption capacity of 2.1wt% with a wide and flat equilibrium plateau pressure, while specimen B and specimen C have desorption capacities of 0.6wt% and 0.3wt%, respectively, with narrow and tilted plateau pressures.

The hydrogen storage performance of V-based alloy depends strongly on the impurity content. As the nitrogen and oxygen impurities are introduced together, two possible effects should be taken into consideration. The effect of interstitial nitrogen on hydrogen storage performance in vanadium-based alloys was investigated in Ref.[

23]. When the N content is 0.4wt%, the maximum hydrogen content is slightly decreased by approximately 6%, compared with that of pure V material. Yamanaka et al^{[Reference 24

Baidu Scholar}24] found that adding 0.7wt% nitrogen in the V_0.37Ti_0.33Mn_0.30 alloy results in a slight decrease in maximum hydrogen content by about 5% at the first cycle. In this research, the content of nitrogen is less than 0.3wt%. A small amount of nitrogen is accumulated in the secondary phase, and the majority of nitrogen is mainly bound with titanium, which causes the composition change of the main phase and therefore reduces the lattice parameter. It is inferred that the dissolved nitrogen has an negligible effect on the maximum hydrogen storage capacity, i.e., the dissolved nitrogen has the secondary effect on the hydrogen absorption properties of V-based alloys. It is reported that even a small amount of oxygen can deteriorate the hydrogen storage capacity of V-based alloy^{[Reference 20-22}20-22]. Ref.[25] reports that the oxygen atoms are on the octahedral interstice in bcc lattice. The normal hydrogen absorption sites are decreased as the oxygen atoms occupy the interstitial sites. Furthermore, the hydrogen diffusion is suppressed strongly as a result of direct interaction between oxygen and hydrogen. The formation of the dihydride phase is hindered by the dissolved oxygen and its content is decreased with increasing the oxygen impurities, which is the primary effect of impurity content on the hydrogen absorption properties. Asano et al^{[Reference 22

Baidu Scholar}22] explained the inhibiting effect from the perspective of binding energy of the O-H pair. Thus, the dissolved oxygen has a significant effect on the hydrogen storage capacity of V-based alloy.

3 Conclusions

1) The hydrogen absorption/desorption capacity of V₄₀Ti₂₆Cr₂₆Fe₈ alloys is decreased sharply with increasing the nitrogen and oxygen impurities introduced during the manufacturing process. At 298 K, the alloy with less nitrogen and oxygen has excellent hydrogenation-dehydrogenation kinetics with a maximum hydrogen absorption capacity of 3.94wt% and desorption capacity of 2.1wt%. While the alloy with a relatively large amount of nitrogen and oxygen only has a maximum hydrogen absorption capacity of 1.58wt% and desorption capacity of 0.3wt%.

2) The primary effect mechanism of nitrogen and oxygen contents on the hydrogenation properties is that the oxygen dissolved in the V-based alloys hinders the normal hydrogen occupation and suppresses the formation of the dihydride phase, resulting in the decrease in the hydrogen capacity. The formation of the nitrogen-titanium-enriched secondary phase induces a composition change of the main phase, resulting in the decrease in lattice parameter, which is the secondary effect.

3) The nitrogen-titanium-enriched secondary phase acts as a heterogeneous crystal nucleus during the cooling and crystallization process, and promotes the nucleation of the V₄₀Ti₂₆Cr₂₆Fe₈ alloy. The secondary phase reduces the grain sizeof alloys as a grain refiner.

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English

Effect of Nitrogen and Oxygen Impurities on Chemical Composition, Microstructure, and Hydrogen Storage Performance of V₄₀Ti₂₆Cr₂₆Fe₈ Alloy PDF

Abstract

Keywords

1 Experiment

2 Results and Discussion

2.1 Microstructure and micro-area chemical composition analysis

2.2 Metallographic structure analysis

2.3 Crystal structure of alloy before and after hydrogena-tion

2.4 Performance of hydrogen absorption and desorption

3 Conclusions

References

首 页

期刊简介

编委会

投稿指南

过刊浏览

联系我们

English

Effect of Nitrogen and Oxygen Impurities on Chemical Composition, Microstructure, and Hydrogen Storage Performance of V40Ti26Cr26Fe8 Alloy PDF

Abstract

Keywords

1 Experiment

2 Results and Discussion

2.1 Microstructure and micro-area chemical composition analysis

2.2 Metallographic structure analysis

2.3 Crystal structure of alloy before and after hydrogena-tion

2.4 Performance of hydrogen absorption and desorption

3 Conclusions

References

首页

Effect of Nitrogen and Oxygen Impurities on Chemical Composition, Microstructure, and Hydrogen Storage Performance of V₄₀Ti₂₆Cr₂₆Fe₈ Alloy PDF