目录结构

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参考文献

1 Introduction▲
2 Experimental▲
3 Results and discussion▲
3.1 XRD analysis
3.2 DSC analysis
3.3 Magnetic properties analysis
4 Conclusion▲
图表▲

Rare Metals2018年第3期

Flow hydrogen absorption of LaFe_10.9Co_0.8Si_1.3 compound under constant low hydrogen gas pressure

Bin Fu Jun He Jie Han Jie Hu Li-Wei Pang

Tianjin Key Laboratory for Photoelectric Materials & Devices,School of Materials Science and Engineering,Tianjin University of Technology

Division of Functional Material Research,Central Iron and Steel Research Institute

College of Science,Tianjin University of Technology

School of Materials Science and Engineering,Shijiazhuang Tiedao University

收稿日期：17 November 2017

基金：financially supported by the Tianjin Research Program of Application Foundation and Advanced Technology (No.14JCQNJC04000);the National Key Research and Development Program of China (No.2017YFB0702700);the Hebei Provincial Education Department Project (No.ZD2017066);

Flow hydrogen absorption of LaFe_10.9Co_0.8Si_1.3 compound under constant low hydrogen gas pressure

Bin Fu Jun He Jie Han Jie Hu Li-Wei Pang

Tianjin Key Laboratory for Photoelectric Materials & Devices,School of Materials Science and Engineering,Tianjin University of Technology

Division of Functional Material Research,Central Iron and Steel Research Institute

College of Science,Tianjin University of Technology

School of Materials Science and Engineering,Shijiazhuang Tiedao University

Abstract：

The hydrogen absorption of the LaFe_10.9-Co_0.8Si_1.3 compound under constant 1.01 × 10⁵ Pa H₂ gas in a flow hydrogen atmosphere was studied. The effects of hydrogen absorption on structure, Curie temperature, phase transition and magnetic property were investigated by X-ray diffraction(XRD), differential scanning calorimeter(DSC) and superconducting quantum interference device,respectively. The hydrides of LaFe_10.9Co_0.8Si_1.3 crystallize into NaZn₁₃-type structural phase after hydrogen absorption at temperature from 548 to 623 K. Lower hydrogen absorption temperature is of no advantage for pure 1:13 phase formation in a flow H2 atmosphere. The Curie temperature(T_C) of LaFe_10.9Co_0.8Si_1.3 compound increases by70 K or more after hydrogen absorption. For LaFe_10.9-Co_0.8Si_1.3H_1.8 compound, the maximum magnetic entropy change and the relative cooling power under a magnetic field change of 0-2 T are 6.1 J·kg^-1·K^-1 and 170 J·kg^-1,respectively. Large refrigerant capacity, low hysteresis loss and wide temperature span of magnetic entropy change peak make it a competitive practical candidate for magnetic refrigerant.

Keyword：

La(Fe_xSi_1-x)₁₃ compounds; Magnetic refrigerant; Hydrogen absorption; Magnetic entropy change;

Author： Bin Fu e-mail:fubin@tjut.edu.cn;

Received： 17 November 2017

1 Introduction

Magnetic refrigeration based on magnetocaloric effect(MCE) is considered to be a promising cooling method due to its energy efficiency and green environment properties in comparison with the conventional gas compression method [ 1, 2, 3, 4, 5, 6, 7] .Experimental investigation into magnetic refrigerator shows that the La(Fe_xSi_1-x)₁₃-based compounds with the itinerant-electron metamagnetic (IEM) transition are a kind of magnetic refrigerant with very promising practical future [ 8, 9, 10, 11, 12, 13] .But La(Fe_xSi_1-x)13 compounds are not available for room-or higher-temperature environment because of their lower Curie temperature (T_C).To realize application in room and high temperatures,the improvement of their Curie temperatures is very necessary.It has been demonstrated that T_C can be increased by introducing Co or interstitial atoms such as H into La(Fe_xSi_1-x)13 main phase [ 14, 15, 16, 17] .After hydrogen absorption in a H₂ gas pressure,5.05×10⁶ Pa for example,the Curie temperature greatly increases up to about 280-330 K [ 18, 19, 20] .But the work in this aspect is mainly aimed at hydrogen absorption under a high H₂ pressure from 5.05×10⁵ to5.05×10⁶ Pa.Few studies are concerned about the hydrogen absorption under low H₂ pressure.On the other hand,according to prior studies,typical La(Fe_xM_1-x)13(M=Si and Al) magnetic materials often undergo a firstorder magnetic phase transition [ 21, 22] .That means their magnetic phase transitions always produce stronger hysteresis loss,which results in a significant reduction in refrigerant capacity [ 23] .It is also shown that the magnetic phase transition is driven from the first order to the second order due to partial substitution of Co for Fe [ 9] .Therefore,Co substitution and hydrogen absorption create the possibility for La(Fe_xSi_1-x)13-based materials with less hysteresis loss as room-and high-temperature magnetic refrigerants.

By fully considering practical application,hydrides of LaFe_10.9Co_0.8Si_1.3 in a low hydrogen pressure were prepared by using a flow hydrogen absorption way at different temperatures.The influences of interstitial hydrogen on structure,phase transition and magnetic refrigeration properties were introduced in this article.

2 Experimental

The LaFe_10.9Co_0.8Si_1.3 as-cast samples were fabricated from high pure elements by arc-melting method in argon atmosphere.Depending on burning loss,a certain amount of excess La and Si was added.All the ingots were remelted three or more times to ensure their homogeneity.The ingots were annealed at 1373 K for 240 h in high vacuum and then cooled down to room temperature by quenching in ice water.The whole process of hydrogen absorption was performed in a furnace equipped with a gas supply apparatus.To ensure a good contact with hydrogen,the block samples were removed from oxide scale and crushed into particles with about 0.8 mm in size.After that the particles were kept in an unsealed tube with hydrogen gas flow,which was placed in the furnace chamber.The particles were heated to different temperatures from 523 to 623 K,while hydrogen was induced through the tube continuously.By utilizing this system,a stable 1.01×10⁵ Pa hydrogen pressure was maintained around the samples in the course of hydrogen absorption.To determine the duration of hydrogen absorption,time dependence of hydrogen content was observed as illustrated in Fig.1.The curves show that hydrogen content increases quickly in the initial stage,and then,hydrogen absorption is nearly saturated after about 45 min at 523 and623 K,respectively.About 75 min later,the saturation is reached.As time proceeds,shape of the curve starts to decline slightly after about 100 min,which means that an amount of hydrogen atoms in hydride samples has a slightly decreasing tendency.It indicates that a process of releasing hydrogen begins to occur in the hydrogenated samples at high hydrogen absorption temperature for too long time.According to this phenomenon,the duration of 80 min was used as the hydrogen absorption time in this article.

The content of hydrogen was measured by traditional weighing method:The weights of samples before and after hydrogen absorption were got by electronic scale.The difference of the two values is the weight of hydrogen atoms.So the precise content of hydrogen in the hydride is the ratio of the weight and relative atomic mass of hydrogen.The phase purity and crystal structures of the parent alloy and corresponding hydrides were determined by powder X-ray diffractometer (XRD,Rigaku Dmax-RB)using CuKαradiation.The phase transition was carried out by differential scanning calorimeter (DSC,SII DSC6220).The magnetic measurements were taken by the superconducting quantum interference device (SQUID,Quantum Design MPMS-7).

Fig.1 Time dependence of hydrogen content for hydrogen absorp-tion of LaFe_10.9Co_0.8Si_1.3 compounds at 523 and 623 K

3 Results and discussion

3.1 XRD analysis

Figure 2 presents XRD patterns of LaFe_10.9Co_0.8Si _1.3 compound and its hydrides.The main phase in LaFe_10.9Co_0.8Si_1.3is a cubic NaZn₁₃-type structure,attached by a small amount ofα-Fe (indicated by symbol^*).Although the content ofα-Fe increases after hydrogen absorption due to the unsealed hydrogen absorption environment,hydrides of LaFe_10.9-Co_0.8Si_1.3 remain cubic NaZn₁₃-type structure until the sample was hydrogenated at 523 K.As an example,Fig.3gives the comparison of the (422) peaks in XRD patterns.Compared with original compound LaFe_10.9Co_0.8Si _1.3,the diffraction peaks of LaFe_10.9Co_0.8Si_1.3H_y (y=1.8-2.86)shift toward low angles.It indicates that the introduction of interstitial hydrogen atoms leads to the expansion of unit cell volume.The values of lattice constant and volume expansibility are found in Table 1.

As hydrogen absorption of the La(Fe_xSi_1-x)₁₃ compound is an exothermic reaction [ 24] ,compared to the high temperature,lower temperature is more conducive to hydrogen absorption.Therefore,more hydrogen atoms enter and stay in the crystal lattice with hydrogen absorption temperature decreasing.This operation dramatically causes the lattice distortion and micros true tural change.As shown in Fig.2,the samples hydrogenated at 573-623 K still exhibit the NaZn₁₃-type phase structures.However,with the hydrogen absorption temperature decreasing,splits of characteristic peaks (such as (420),(422) and(531)) appear obviously.The NaZn₁₃-type structure is destroyed in sample of 523 K.This phenomenon indicates that lower hydrogen absorption temperature is of no advantage for pure 1:13 phase formation in a flow H₂atmosphere.It is very important to control hydrogen absorption temperature in order to ensure the formation of NaZn₁₃-type structure.

Fig.2 XRD patterns of LaFe_10.9Co_0.8Si_1.3 compounds before and after hydrogen absorption,where symbol^*refering toα-Fe and T_HAmeaning hydrogen absorption temperature

Fig.3 XRD patterns of (422) peaks of LaFe_10.9Co_0.8Si_1.3H_y (y=0,1.8,2.26,2.74,2.86) compounds

3.2 DSC analysis

The DSC curves of the hydrogenated samples and parent compound LaFe_10.9Co_0.8Si_1.3 are exhibited as shown in Fig.4.It is found that the endothermic peak of LaFe_10.9Co_0.8Si_1.3 shifts to higher-temperature region as the sample was hydrogenized in H₂ gas at 623 and 548 K,respectively,implying the increase in transition temperature of LaFe_10.9Co_0.8Si_1.3.The comparison between curves before and after hydrogen absorption shows that the endothermic peak strength weakens while the span increases.It indicates that phase transition tends toward second order after hydrogen absorption.

Figure 5 shows the differential DSC (DDSC) curves by which the Curie temperature T_C can be obtained.After hydrogen absorption at temperatures from 548 to 623 K,T_C of LaFe_10.9Co_0.8Si_1.3 compound rises remarkably(Table 1).In La(Fe_xSi_1-x)₁₃H_y,interstitial hydrogen atoms occupy the 24d sites.With the introduction of interstitial hydrogen atoms,distance between Fe^Ⅱ-Fe^Ⅱatoms which occupy 96i sites is expanded,leading to lattice expansion which produces a substantial rise in Curie temperature [ 24] .With hydrogen absorption temperature changing,interstitial hydrogen atoms in LaFe_10.9Co_0.8Si_1.3 cause volume expansion to a great extent,which results in significant increases of T_C.It should be emphasized that it exhibits an apparent upward trend of Curie temperature as the hydrogen absorption temperature decreases.

Different from the constant H₂ gas flow,the gradual decrease in hydrogen pressure is performed during the process of hydrogen absorption under 1.01×10⁵ Pa pressure in a seal furnace in early literature [ 24] .It will lead to the uncertainty of the Curie temperature change after hydrogen absorption [ 24] .In this article,the regular change of Curie temperature of hydrides is easily realized by only maintaining the constant hydrogen pressure.This simple experimental method can be regarded as an effective technical approach to modulate phase transition temperature of La(Fe,Si) 13 compounds.

On the other hand,previous experimental result [ 18, 19, 20] about high-pressure hydrogen absorption shows that once the high-pressure H₂ gas atmosphere is employed during hydrogen charging operation,the interstitial hydrogen atoms will be distributed homogeneously in unit cell of La(Fe_xSi_1-x)₁₃,which causes highly symmetric lattice expansion.In those hydrides,the itinerant-electron metamagnetic (IEM) transition characteristic is maintained [ 18, 19, 20] .On the contrary,the hydrides obtained under a relatively much lower pressure of 1.01×10⁵ Pa H₂ gas will exhibit the interstitial hydrogen atomic site occupation with different 24d sites in each unit cell.It finally leads to an asymmetrical hydrogen absorption and expands the phase transition temperature range [ 24] .Furthermore,the asymmetrical hydrogen absorption has the power of weakening first-order phase transition in LaFe_10.9Co_0.8Si_1.3compound.Compared with DSC curve of LaFe_10.9Co_0.8-Si₁₃H_1.8 in Fig.4,the latent heat of LaFe_10.9Co_0.8Si_1.3H_2.86continues to diminish,which means that the second-order phase transition is further enhanced.The tendency of second-order transition in the hydrides will decrease hysteresis loss and then increase the efficiency of magnetic refrigerator performance [ 25] .Also,the wider DSC peak implies that the hydrides of LaFe_10.9Co_0.8Si _1.3 compound are one of the very promising candidates to be applied in a broader cycle refrigeration temperature range.

下载原图

Table 1 Hydrogen absorption temperatures,lattice constants (a),unit cell volumes (V),volume expansibility (ΔV/V) and Curie temperatures(T_C) of LaFe_10.9Co_0.8Si _1.3 and compounds after hydrogen absorption

Fig.4 DSC measurements of LaFe_10.9Co_0.8Si_1.3,LaFe_10.9Co_0.8-Si_1.3H_1.8 and LaFe_10.9Co_0.8Si_1.3H_2.86 compounds,whereΔQ refering to latent heat

Fig.5 DDSC measurements of LaFe_10.9Co_0.8Si_1.3,LaFe_10.9Co_0.8-Si_1.3H_1.8 and LaFe_10.9Co_0.8Si_1.3H_2.86 compounds

3.3 Magnetic properties analysis

In order to confirm the phase transition properties,the magnetization curves (M-H curves) of LaFe_10.9Co_0.8-Si_1.3H_1.8 compound were measured,as shown in Fig.6a.LaFe_10.9Co_0.8Si_1.3H_1.8 exhibits a magnetization curve with no clear IEM transition characteristic.In Fig.6b,the Arrott curves calculated from M-H curves show neither negative slope nor inflection point,which clearly reflects the secondorder phase transition of LaFe_10.9Co_0.8Si_1.3H_1.8 [ 26] .The inset of Fig.6a shows the M-H curves measured at Curie temperature of LaFe_10.9Co_0.8Si_1.3H_1.8 with field increasing and decreasing.It is noteworthy that the curves almost show no hysteresis effect in magnetic field loop.This performance with small magnetic hysteresis is a very important characteristic which is favorable to magnetic refrigeration application.

The value of magnetic entropy change (ΔS_m) is calculated from the Maxwell relation shown as follows:

The magnetic entropy change of LaFe_10.9Co_0.8Si_1.3H_1.8compound for magnetic field change of 0 to 2 T is shown in Fig.7.The maximum values of|ΔS_m|are about6.1 J·kg^-1·K^-1.It is worthwhile to note that the|ΔS_m|of LaFe_10.9Ca_0.8Si_1.3H_1.8 is larger than that of Gd,which has a|ΔS_m|value of 5.0 J·kg^-1·K^-1 at T_C=294 K for a field change of 0 to 2 T [ 27] .Therefore,the LaFe_10.9Co_0.8Si_1.3H_1.8 compound is a potential material as magnetic refrigerant in an extended temperature region around room temperature.

Fig.6 a M-H curves and b Arrott curves of LaFe_10.9C0_0.8Si_1.3H_1.8 compounds (inset showing magnetization curve measured at 346 K in field-increasing and field-decreasing modes)

In fact,relative cooling power (RCP) is also an important parameter when the magnetic refrigerants are used in practical cooling system.It is given by the product of|ΔS_m|and full width at half-maximum (δT) [ 28] ,which is calculated as follows:

Then,the calculated RCP of LaFe_10.9Co_0.8Si_1.3H_1.8compound is about 170 J·kg^-1 under the magnetic field change of 0-2 T.This value is almost the same as Gd(～169 J·kg^-1) [ 29] and larger than some typical magnetic refrigerants such as MnFeP_0.45As_0.65 (～147 J·kg-1) [ 27] and La(Fe₀₈₈Si_0.12H_1.5 (～147 J·kg^-1) [ 30] .Large RCP values indicate that LaFe_10.9Co_0.8Si_1.3H₁₈ compound will exhibit high refrigerant capacity for actual use.

Fig.7 Magnetic entropy changes of LaFe_10.9Co_0.8Si_1.3H_1.8 in mag-netic field of 0-2 T

4 Conclusion

The hydrogen absorption in 1.01×10⁵ Pa flow hydrogen atmosphere was investigated for the LaFe_10.9Co_0.8Si_1.3compound.Under the constant hydrogen pressure,even though the T_C increases with hydrogen absorption temperature decreasing,lower temperature is of no advantage for phase formation of LaFe_10.9Co_0.8Si_1.3 compound.The hydrides of LaFe_10.9Co_0.8Si_1.3 compound crystallize into NaZn₁₃-type structure after hydrogen absorption at the temperature higher than 548 K.The regular changes of T_Cwith hydrogen absorption temperature provide an effective method to modulate phase transition temperature of the La(Fe_xSi_1-x) 13 compounds.After the hydrogenation,the temperature region of the magnetic phase transition becomes wider and the second-order phase transition enhances.The MCE and RCP in LaFe_10.9Co_0.8Si_1.3H_1.8compound are stronger than those of many typical magnetic refrigerants,such as Gd and La(Fe_0.88Si_0.12)13H_1.5.The material with high refrigerant capacity and broader work temperature range offers the great possibility to become the attractive candidate for application in the magnetic refrigeration technology near room temperature.

Acknowledgements This study was financially supported by the Tianjin Research Program of Application Foundation and Advanced Technology (No.14JCQNJC04000),the National Key Research and Development Program of China (No.2017YFB0702700) and the Hebei Provincial Education Department Project (No.ZD2017066).