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Effect of malonic acid and of different doping methods on the superconducting properties of MgB2 superconductors
Accepted Manuscript
Effect of malonic acid and of different doping methods on the superconducting properties of MgB2 superconductors Chunyan Li , Hongli Suo , Min Liu , Lin Ma , Yi Wang , Min Tian , Baicen Wan , Jin Cui , Yaotang Ji PII: DOI: Reference:
S0921-4534(18)30184-9 https://doi.org/10.1016/j.physc.2018.10.011 PHYSC 1253406
To appear in:
Physica C: Superconductivity and its applications
Received date: Revised date: Accepted date:
5 May 2018 16 October 2018 19 October 2018
Please cite this article as: Chunyan Li , Hongli Suo , Min Liu , Lin Ma , Yi Wang , Min Tian , Baicen Wan , Jin Cui , Yaotang Ji , Effect of malonic acid and of different doping methods on the superconducting properties of MgB2 superconductors, Physica C: Superconductivity and its applications (2018), doi: https://doi.org/10.1016/j.physc.2018.10.011
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Highlights
The malonic acid was proved to be an effective carbon source.
The carbon atoms were released by the reaction of Mg and CO2.
The x value in Mg(B1-xCx)2 increased with a higher amount of malonic acid addition. A moderate amount of malonic acid addition resulted in grain refinement.
Bulk MgB2 with 10 wt% malonic acid addition showed the highest Jc value.
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Effect of malonic acid and of different doping methods on the superconducting properties of MgB2 superconductors Chunyan Li, Hongli Suo, Min Liu, Lin Ma, Yi Wang, Min Tian, Baicen Wan, Jin Cui, Yaotang Ji
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Key Laboratory of Advanced Functional Materials, Ministry of Education, College of Materials Science and Engineering, Beijing University of Technology, 100 Pingleyuan, Chaoyang District, Beijing 100124, China
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Corresponding author: E-mail: [email protected]
Abstract:
In this paper, malonic acid was tested as a new carbon source and its effects on the structural and superconducting properties of bulk MgB2 have been systematically
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investigated. The detailed pyrolysis process of malonic acid was analyzed to understand the possible doping mechanism of malonic acid added MgB2 samples. The
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weight fraction of MgO phase and the actual carbon substitution level in bulk MgB2 increased with a higher amount of malonic acid addition, which was proven to be
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mainly caused by the reaction of Mg and CO2 gas. A moderate amount of malonic acid addition resulted in grain refinement and enhanced flux pinning. Bulk MgB2 with
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10 wt% malonic acid addition showed much higher Jc values in the entire magnetic field range. Three different doping methods with the same amount of malonic acid
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addition were compared. MgB2 samples using malonic acid and ethanol processed boron showed the highest Jc value, which was 7.2 times larger than that of the pure MgB2 at 20 K and 5 T.
Keywords: MgB2, malonic acid, decomposition, doping, critical current density
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1. Introduction MgB2 superconductors have many advantages such as low cost, light weight, large coherence length, small anisotropy and a relatively high transition temperature (Tc = 39 K), which render MgB2 competitive with respect to other superconductors for magnet application at 20-25 K [1]. However, the in-field critical current density (Jc) of MgB2 still needs to be further improved in view of practical large-scale applications.
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Various methods including chemical doping [2], irradiation [3], cold high-pressure densification (CHPD) [4] and hot isostatic pressure (HIP) [5] have so far been tried. Carbon doping has been proved to be one of the most efficient methods, since the C substitution for B in the MgB2 lattice will increase electron scattering, lower the
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electronic mean free path and hence improve the upper critical magnetic field (Bc2) and high-field Jc performance of MgB2 superconductors [6].
Until now, mainly three types of carbon source have been widely studied. (1) Nano-sized carbon source (SiC, C60 [7], carbon nanoparticles [8], nano-diamond [9],
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graphene [10], etc.). A significant improvement of high-field Jc and a slight reduction of low-field Jc in nano-SiC doped MgB2 has firstly been found by Prof. Dou’s group
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[11]. Sintering temperatures of ≧ 900 °C are needed for a better reaction in nano-carbon and carbon nanotube (CNT) doped MgB2 superconductors [12-14]. It
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was observed that nano-sized carbon sources are easy to agglomerate during the solid-solid mixing (SSM) with Mg and B powders due to the large surface energy. It
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is thus a great challenge to obtain homogeneous carbon distribution and substitution in nano-carbon source doped MgB2 superconductors [15].
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(2) Carbohydrates (sugar [16], malic acid [17-19], urea [20], glycerin [21],
glycine [22-24], etc.). In contrast to the nano-carbon source, carbohydrates are soluble in solvents so that the solution can mix with boron powders uniformly. Boron powders coated with carbohydrates or carbon can be obtained after heating the mixture of carbohydrate, boron powders and solvent at different temperatures [25, 26]. MgB2 with carbohydrates doping obtained by “wet” mixing showed a significant in-field Jc enhancement compared with that of the pure MgB2. However, there is so far almost no report comparing the doping effects of “wet” mixing and “dry” mixing 3
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(SSM method). (3) Hydrocarbon (benzene [27], ethyltoluene [28], pyrene [29], coronene [30], etc.). In contrast to carbohydrates, hydrocarbons have no oxygen, which will not introduce a large amount of MgO impurities into the MgB2 matrix. Maeda et al [31] discovered considerable in-field Jc improvement by pyrene (C16H10) gas diffusion into highly dense MgB2. Ye et al [32] used coronene (C24H12) as the carbon source. The 3
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nm coronene coating and 4 nm amorphous carbon coating were formed on nano-sized boron powders by heating the mixture of coronene and boron powders at 520 and 630 °C, respectively. The highest Jc value of 1.07×105 A/cm2 was achieved in coronene doped MgB2 wires at 4.2 K and 10 T [33]. In addition to hydrocarbons,
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some other carbon-containing materials may also be selected to process boron powders. Recently, a uniform coating of amorphous carbon on boron powders was obtained through the direct pyrolysis of PMMA polymer ((C5O2H8)n) at 500 and 800 °C [34].
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In the present study, low-cost malonic acid (C3H4O4) was first examined as a carbon source for improving the Jc performance of MgB2. We tried three methods of
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malonic acid addition: (1) “dry” mixing (SSM method), (2) “wet” mixing and vacuum drying at 60 °C (60VD method), and (3) “dry” mixing and sealed heat treatment at
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250 °C (250SH method). In addition, the decomposition behavior of malonic acid and the correlation between different doping amounts, doping methods, lattice parameters,
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microstructure and superconducting properties were investigated in detail.
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2. Experimental Mg
powders
(Alfa
Aesar,
325
mesh,
99.8%) ， amorphous
boron
(Pavezyum, >98.5%) and malonic acid (Alfa Aesar, 99.5%) were weighted in the ratio of MgB2 + y wt% (y = 0,5,10,15 and 30). All the powders were well mixed in an agate mortar and the mixture was pressed into pellets under the 3-ton uniaxial pressure. All the processes mentioned above were conducted in the glove box. The pellets were put inside the thick plastic bags and the bags were vacuumed and sealed using a sealing machine. The sealed plastic bags with pellets were put inside 4
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the oil of a hydraulic machine and then re-pressed under the 200 MPa isotropic pressures. The pellets were wrapped inside the Ta foils without sealing and then sintered at 800 °C for 3 hours in a tube furnace under Ar/H2 (4%) gas flow. This fabrication method was named as solid-solid mixing (SSM) method in the present work. The bulk MgB2 samples were ground into powders. The phase identification
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of the powder samples was performed by x-ray diffraction (Bruker D8 Advance). The PDXL2 software was used to calculate the full widths at half maximum (FWHM) values of the different diffraction peaks and the weight fraction of the MgO phase in the MgB2 matrix. The microstructures of the samples were examined by field
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emission scanning electron microscopy (FESEM). All the samples were cut to the size of a×b×c mm3 from as-sintered pellets. Zero field cooled (ZFC) magnetization versus temperature (M-T) and magnetization hysteresis (M-H) loops were obtained by using a quantum design vibrating sample magnetometer option (PPMS, Quantum Design).
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The Jc values were determined using the Bean model, Jc = 20ΔM/va(1−a/3b), where ΔM (emu) is the vertical width of the M-H loop, v is the volume of the measured
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sample, a and b are the cross-sectional sizes of the sample perpendicular to the applied field with b > a.
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According to the experimental results, among all the bulk MgB2 fabricated through SSM method, the 10 wt% malonic acid added MgB2 showed the highest Jc
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value. Hence, in order to analyze the effect of different doping methods on the superconducting performance of Mg(B1-xCx)2, the same amount of malonic acid
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addition (10 wt%) was used to pre-treat the boron powders. Two different methods were used. In study 1, the additive was dissolved in ethanol solution, mixed with boron powder, and then vacuum dried at 60 °C for 1 hour. The treated powder was mixed with Mg powder. The powder mixture was pressed and sintered under the same condition as that used for the SSM method. The obtained sample was named as 10 wt%-60VD (60VD for abbreviation). In study 2, the malonic acid was mixed with boron powder and pressed into pellets. The compressed mixtures were sealed in a vacuum silica tube and then heated at 250 °C for 1 hour. The pellets were then 5
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reground and mixed with Mg powder, followed by the same process used for SSM method. The obtained sample was named as 10 wt%-250SH (250SH for abbreviation). These samples were also characterized using the same methods as SSM samples.
3. Results and discussion 3.1 Thermal decomposition of the malonic acid
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It’s important to analyze the thermal decomposition process of malonic acid under high-purity argon atmosphere. Thermogravimetry (TG) and differential scanning calorimetry (DSC) analyzer NETZSCH STA 449C was used to measure the thermal properties of the malonic acid. The TG-DSC curve of malonic acid is shown in Fig. 1.
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A weight loss of 99 % is observed in the range of 120 to 200 °C. The endothermic peaks centered at 135 and 175 °C correspond to the melt and decomposition of malonic acid, respectively. A similar result was obtained in the TG-DTA curves of malonic acid measured by J. Györe et al. [35] The DTA peaks at 88 and 133 °C were
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recognized as the crystal structure modification and the melting point of malonic acid, respectively. In addition, the malonic acid decomposed at about 165°C according to
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the equation:
HOOC-CH2-COOH
CH3COOH + CO2
(1)
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There is no obvious weight loss between 200 and 650 °C in Fig. 1 since the decomposition products are all in the gaseous state. When the temperature exceeds
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440 °C, the acetic acid (CH3COOH) will start decomposing in two possible ways as
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follows [36]:
CH3COOH
CH4 + CO2
(2)
CH3COOH
CH2CO + H2O
(3)
The ketene (CH2CO) can decompose into C2H4 and CO. The C2H4 is likely to decompose to CH4 and C, but CH4 is very difficult to decompose into C and H2 below 1000 °C without the catalyst or high pressure [37]. It seems that the gaseous products of malonic acid cannot turn into nano-carbon efficiently. However, the gaseous products still affect the lattice structure and superconducting properties of MgB2, 6
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which will be discussed below.
Fig. 1. TG-DSC curve of malonic acid powder
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3.2 SSM method
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Fig. 2 shows the XRD patterns of MgB2 for different levels of malonic acid addition. All samples consist mainly of MgB2 and a less amount of MgO impurity.
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The relative intensity of the MgO peak increases monotonically with the adding amount of malonic acid. The same result can also be seen in Fig. 3(a) comparing the
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calculated weight fraction values of MgO phase in the MgB2 matrix. In particular, the MgO content of 30 wt% malonic acid added sample is about 3 times larger than that
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of the pure MgB2 sample. Since an isotropic pressure of 200 MPa was applied to the bulk mixture of Mg, B and malonic acid, part of oxygen-containing decomposition products of the malonic acid, including CO2, CO and H2O, may be sealed inside the bulk before the formation of the MgB2. It’s also very possible that the CO2 gas inside the compact bulk is not fully released out, because it can react with magnesium according to the reaction equation: 2Mg + CO2
2 MgO + C
(4)
This reaction was also confirmed in another carbohydrate - glycine doped MgB2 bulks 7
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by Cai et al through a thermodynamic and kinetic analysis of the MgO formation mechanism [24]. The carbon atoms generated from this reaction can replace B atoms in the formation process of MgB2. In addition, the H2O vapor may also contribute to
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the formation of the MgO.
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Fig. 2. XRD patterns of pure and malonic acid added MgB2 samples
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The FWHM values of the (100), (002) and (110) peaks for MgB2 samples with different amount of malonic acid addition are displayed in Fig. 3(b). The three peaks
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show a similar trend. The FWHM values first increase and then decrease with increasing amount of malonic acid additive. However, the peak broadening observed
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in XRD patterns is mainly related to the crystallite size and lattice strain except for the instrumental contribution. As demonstrated in many past studies, the broadening of the peaks in carbon source doped MgB2 could be explained by carbon doping induced grain size refinement [25] and by the lattice distortion (degradation of the crystallinity) originating from the C substitution for B [38]. Hence, the significant broadening of the peaks observed in malonic acid added MgB2 samples may also indicate smaller crystallite size or increased lattice distortion. 8
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MgO
a
8
6
4
2
0
5
10
15
20
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Weight fraction (%)
10
25
30
Amount of added malonic acid (wt %)
(100) (002) (110)
b
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0.6
0.4
0.3
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FWHM (deg.)
0.5
0.1
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0.2
0
5
10
15
20
25
30
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Amount of added malonic acid(wt%)
Fig. 3. (a) Weight fraction of MgO phase, (b) FWHM values of different diffraction peaks for the
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pure and different amount of malonic acid added MgB2 samples
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Scanning electron microscope (SEM) images of the pure and malonic acid added bulk MgB2 samples are presented in Fig. 4. The plate-shaped MgB2 grains are randomly distributed. The grains of the malonic acid added samples appear to be thinner and smaller than those of the pure MgB2. The grain size refinement is probably due to the impurity particles introduced by the doping, which serve as additional nucleation sites for grain formation. The additional impurity particles and, thus, nucleation sites can restrict the mobility of grain walls, preventing them from growth [25]. It can also be seen that the 15 wt% malonic acid added sample shows 9
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some cracks, 30 wt% added sample has large-sized pores, which will all result in poor connectivity. This indicates that overdoping induced large amount of gas releasing and MgO impurities will destroy the connectivity of MgB2.
(b)
5 μm
5 μm
(d)
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5 μm
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(e)
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(c)
5 μm
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(a)
5 μm
Fig. 4. SEM images of MgB2 + x wt% malonic acid samples with (a) x = 0; (b) x = 5; (c) x = 10; (d) x = 15; (e) x=30.
The lattice parameters a and c of MgB2 were calculated using the (110) and (002) peak positions, respectively. The actual level of C substitution in doped MgB2 was estimated through the relation: x = 7.5 × Δ(c/a) [39]. Fig. 5 shows the effect of the malonic acid addition on the structural parameters. As the malonic acid additions 10
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increase from 0 to 30 wt%, the a value decreases. In contrast, both c/a and x values of Mg(B1-xCx)2 increase. This result further confirms that C atoms produced by the reaction of Mg and CO2 indeed replaced the B atoms in Mg(B1-xCx)2 samples.
(a)
3.084
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a-axis (Å)
3.086
3.082 3.080 3.078
(b)
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c/a
1.145 1.144 1.143
0.01
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(c)
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0.02
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x of C in Mg(B1-xCx)2
1.142
0
10
20
30
Amount of added malonic acid (wt%)
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0.00
Fig. 5. (a) a-axis lattice parameter, (b) c/a value, and (c) actual carbon substitution level (x) in Mg(B1-xCx)2 for pure and different amount of malonic acid added MgB2 samples
The normalized zero field cooled (ZFC) magnetization versus temperature for the pure and malonic acid added bulk MgB2 are shown in Fig. 6. Compared with pure MgB2, a slight Tc decrease is observed for malonic acid added samples. Especially for 30 wt% added MgB2, the Tc value is about 37.03 K, which is 1.5 K lower than that of 11
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pure MgB2 due to carbon substitution induced impurity scattering. It is well known that grain connectivity, impurity and homogeneity in the MgB2 matrix will affect the superconducting transition width (ΔT). From M-T curves, it is seen that the ΔT values of MgB2 with 0, 5 and 10 wt% malonic acid are almost similar. However, the 15 wt% added sample shows much bigger ΔT and 30 wt% added MgB2 displays an incomplete superconducting transition curve. This result may be related to the larger
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amount of MgO impurity and the inferior crystallinity, which are both caused by malonic acid overdoping.
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H=20 Oe
-0.2
-0.4
-0.6
ZFC
-0.8
pure 5 wt% 10 wt% 15 wt% 30 wt%
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Normalized Magnetization
0.0
30
35
40
45
50
Temperature (K)
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25
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-1.0
Fig. 6. Temperature-dependent normalized magnetization for pure and malonic acid added MgB2
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samples, measured under zero field cooled condition.
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The field dependence of critical current density for the pure and malonic acid
added MgB2 at 20 K is observed in Fig. 7. The Jc values of 5 and 10 wt% added MgB2 are higher than that of the pure sample at all magnetic fields. The highest Jc value obtained by 10 wt% malonic acid added MgB2 is 5.3 times higher than pure MgB2 at 5 T. One reason for the Jc improvement is the enhanced grain boundary pinning caused by the refinement of grain size. In addition, carbon substitution for B sites induces higher Hc2 and contributes to the improvement of high-field Jc performance. For the 15 wt% malonic acid added MgB2, the low-field Jc is slightly depressed, but 12
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the high-field Jc is still much larger than that of the pure MgB2. Moreover, the 30 wt% added MgB2 shows the lowest Jc at low field, which may be due to a large amount of MgO impurity. However, the slope of the Jc - B curve for 30 wt% added MgB2 is the smallest among all samples, which should be due to the highest actual carbon substitution level (x) in Mg(B1-xCx)2.
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pure 5 wt% 10 wt% 15 wt% 30 wt%
5
4
10
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2
Jc(A/cm )
10
3
10
@ 20K 2
0
1
2
3
4
5
6
M
10
B (T)
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Fig. 7. Critical current density at 20 K vs. applied magnetic field for pure and malonic acid
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added MgB2 samples
The flux pinning force (Fp) is calculated by Fp= Jc × B and shown in Fig. 8(a) for
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pure and malonic acid added MgB2. The Fp values for the 10 wt% added MgB2 are much higher than for other samples in the entire magnetic field range. On the contrary,
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the low-field Fp values for 30 wt% added sample are much lower than those for pure MgB2. This may due to the poor connectivity caused by the large-sized pores and a large amount of MgO impurities. Furthermore, the normalized flux pinning force (Fp/Fp, max) versus the applied field for all samples is compared in Fig. 8(b). At high fields, the Fp/Fp, max values monotonically increase with the amount of malonic acid, which is consistent with the x variation in Mg(B1-xCx)2 shown in Fig. 5(c). This also confirms that carbon substitution for boron plays an important role in improving the high-field Jc performances. 13
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a
pure 5 wt% 10 wt% 15 wt% 30 wt%
3
Fp (N/cm )
1.0x10
8
0.0
0
1
2
3
4
6
pure 5 wt% 10 wt% 15 wt% 30 wt%
b
M
1.0
0.5
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Fp / Fp,max
5
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B (T)
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5.0x10
0.0
1
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0
2
3
4
5
6
B (T)
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Fig. 8. Field dependence of the flux pinning force-Fp (a) and Fp/ Fp, max (b) for pure and malonic acid added MgB2 samples
3.3 60VD and 250SH methods From Table 1, it is seen that the 60VD and SSM samples show very similar lattice parameters, x and FWHM values. However, the weight fraction of the MgO for 60VD sample is a little higher than that for SSM sample. This may be explained by the introduced oxygen during the solution mixing of the malonic acid and nano boron powders in the air. The Fig. 9 shows that the Jc at 20 K, 5 T for the 60VD sample is 14
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7.2 times and 1.4 times higher than those for pure and malonic acid added samples prepared from the SSM method, respectively. It can be observed that the Jc of 60VD sample is higher than that of SSM sample in the entire magnetic field range. This is due to that the 60VD method can achieve a more uniform distribution of the malonic acid inside the boron powders and more homogeneous carbon doping [25]. In addition, the more uniformly distributed MgO and C impurity could contribute to limiting the
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growth of the MgB2 grains, resulting in larger grain boundary pinning. However, the Jc of SSM sample is only slightly lower than that of the 60VD sample. This may indicate that the gaseous carbon source such as CO2 could spread or diffuse within the
Sample ID
Lattice constant (Å)
Mg(B1-xCx)2
c/a
3.0863 1.1419
10 wt%-SSM
FWHM (deg.)
(100)
(002)
(110)
Weight fraction of MgO (%)
0
0.247
0.316
0.343
3.3
3.0811 1.1438
0.0143
0.295
0.341
0.422
4.6
10 wt%-60VD
3.0812 1.1438
0.0140
0.296
0.326
0.422
5.2
10 wt%-250SH
3.0839 1.1428
0.0067
0.270
0.328
0.376
3.75
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Pure MgB2
x value in
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a
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sample, so the SSM sample could also obtain effective doping effect.
Table 1. Lattice parameters, x values in Mg(B1-xCx)2, FWHM, weight fraction of MgO of pure and
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malonic acid added MgB2 samples using different doping methods.
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In order to verify the decomposition behavior of malonic acid, the compressed mixture of boron and malonic acid powders for 250SH sample was pre-treated at
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250 °C for 1 hour as mentioned before. This temperature was chosen to be higher than the decomposition temperature of malonic acid. As a result, all the structural parameters of the 250SH sample are between the values of pure MgB2 and those of SSM and 60VD samples as listed in Table 1. Besides, Fig. 9 shows that the high-field Jc performance of the 250SH sample is higher than for the pure MgB2 but lower than for SSM/60VD samples. These results demonstrated that the pre-heat treatment of compressed mixture at 250 °C followed by the grinding process for obtaining 15
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precursor powder at room temperature leads to a decrease of the actual carbon content in boron powders. This is because at least 1/3 of the C atoms contained in malonic acid can be released in the form of CO2 gas according to chemical reaction formula (1). However, a part of acetic acid still remained inside the boron powders after cooling down to room temperature.
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pure 10 wt%-SSM 10 wt%-60VD 10 wt%-250SH
5
2
Jc (A/cm )
10
4
3
10
@ 20K 2
0
2
M
10
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10
4
6
B (T)
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Fig. 9. The magnetic field dependence of Jc at 20K for pure and malonic acid added MgB2
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samples using different doping methods
It turns out that preparing the uniform nano-carbon coated boron through
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pre-heating the mixture of the malonic acid and amorphous boron powders at higher temperature is a complicated process. In fact, even after pyrolysis of malonic acid at
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1000 °C, approximately 3/4 of the carbon atoms will exist in the form of CO2 and CH4, and these gases will be released from the boron powders. Therefore, to prepare carbon-coated boron using malonic acid as the carbon source, a larger amount of malonic acid additions and appropriate reaction temperatures need to be further studied. On the other hand, malonic acid added MgB2 through SSM and 60VD methods obtained significant Jc improvement in the entire magnetic field range. In particular, the Jc of 60VD sample is 7.2 times higher than that of the pure MgB2 at 20 K and 5 T, thus indicating that malonic acid is an effective carbon source for 16
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enhancing the Jc of MgB2.
4. Conclusion In summary, bulk MgB2 samples with different amount of malonic acid addition were prepared using conventional solid state sintering at 800 °C. With increasing the amount of malonic acid, the lattice parameter a value decreased, while the MgO
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content in the MgB2 matrix increased. A suitable amount of MgO might be able to limit the growth of MgB2 grains. Furthermore, the 10 wt% malonic acid added MgB2 showed the highest Jc and Fp performance in the whole magnetic field range. The effect of three different doping methods on the structural and superconducting
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properties of MgB2 has been investigated. Interestingly, the Jc of SSM sample (made from “dry mixing”) was only slightly lower than 60VD sample (made from “wet mixing”), which could be attributed to the reaction of Mg and CO2 and the good diffusivity of CO2 within the sample.
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Acknowledgment
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This work is financially supported by the National Natural Science Foundation of China (51571002), by the General Program of Science and Technology Development
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Project of Beijing Municipal Education Commission of China (KM201810005010), by the Beijing Municipal Natural Science Foundation (2172008) and by 211 Program
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of Beijing City and Beijing University of Technology.
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[39]
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Journal of Alloys and Compounds. 727 1213-20 Maeda M, et al. 2011 JOURNAL OF APPLIED PHYSICS. 109 Ye S, Song M and Kumakura H. 2015 Nanotechnology. 26 045602 Ye SJ, Matsumoto A, Zhang YC and Kumakura H. 2014 Superconductor Science and Technology. 27 085012 Ranot M, Jang SH, Shinde KP, Sinha BB, Bhardwaj A, Oh Y-S, Kang S-H and Chung K. 2017 Journal of Alloys and Compounds. 724 507-14 GYÖRE J and ECET M. 1970 Journal of Thermal Analysis. 2 397-409 Nguyen MT, Sengupta D, Raspoet G and Vanquickenborne LG. 1995 J Phys Chem. 99 6 Zein SHS, Mohamed AR and Sai PST. 2004 Industrial & Engineering Chemistry Research. 43 4864-70 Hossain MSA, Kim JH, Wang XL, Xu X, Peleckis G and Dou SX. 2007 Superconductor Science and Technology. 20 112-16 Avdeev M, Jorgensen JD, Ribeiro RA, Bud’ko SL and Canfield PC. 2003 Physica C: Superconductivity. 387 301-06
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Effect of malonic acid and of different doping methods on the superconducting properties of MgB2 superconductors Chunyan Li , Hongli Suo , Min Liu , Lin Ma , Yi Wang , Min Tian , Baicen Wan , Jin Cui , Yaotang Ji PII: DOI: Reference:
S0921-4534(18)30184-9 https://doi.org/10.1016/j.physc.2018.10.011 PHYSC 1253406
To appear in:
Physica C: Superconductivity and its applications
Received date: Revised date: Accepted date:
5 May 2018 16 October 2018 19 October 2018
Please cite this article as: Chunyan Li , Hongli Suo , Min Liu , Lin Ma , Yi Wang , Min Tian , Baicen Wan , Jin Cui , Yaotang Ji , Effect of malonic acid and of different doping methods on the superconducting properties of MgB2 superconductors, Physica C: Superconductivity and its applications (2018), doi: https://doi.org/10.1016/j.physc.2018.10.011
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Highlights
The malonic acid was proved to be an effective carbon source.
The carbon atoms were released by the reaction of Mg and CO2.
The x value in Mg(B1-xCx)2 increased with a higher amount of malonic acid addition. A moderate amount of malonic acid addition resulted in grain refinement.
Bulk MgB2 with 10 wt% malonic acid addition showed the highest Jc value.
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Effect of malonic acid and of different doping methods on the superconducting properties of MgB2 superconductors Chunyan Li, Hongli Suo, Min Liu, Lin Ma, Yi Wang, Min Tian, Baicen Wan, Jin Cui, Yaotang Ji
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Key Laboratory of Advanced Functional Materials, Ministry of Education, College of Materials Science and Engineering, Beijing University of Technology, 100 Pingleyuan, Chaoyang District, Beijing 100124, China
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Corresponding author: E-mail: [email protected]
Abstract:
In this paper, malonic acid was tested as a new carbon source and its effects on the structural and superconducting properties of bulk MgB2 have been systematically
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investigated. The detailed pyrolysis process of malonic acid was analyzed to understand the possible doping mechanism of malonic acid added MgB2 samples. The
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weight fraction of MgO phase and the actual carbon substitution level in bulk MgB2 increased with a higher amount of malonic acid addition, which was proven to be
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mainly caused by the reaction of Mg and CO2 gas. A moderate amount of malonic acid addition resulted in grain refinement and enhanced flux pinning. Bulk MgB2 with
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10 wt% malonic acid addition showed much higher Jc values in the entire magnetic field range. Three different doping methods with the same amount of malonic acid
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addition were compared. MgB2 samples using malonic acid and ethanol processed boron showed the highest Jc value, which was 7.2 times larger than that of the pure MgB2 at 20 K and 5 T.
Keywords: MgB2, malonic acid, decomposition, doping, critical current density
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1. Introduction MgB2 superconductors have many advantages such as low cost, light weight, large coherence length, small anisotropy and a relatively high transition temperature (Tc = 39 K), which render MgB2 competitive with respect to other superconductors for magnet application at 20-25 K [1]. However, the in-field critical current density (Jc) of MgB2 still needs to be further improved in view of practical large-scale applications.
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Various methods including chemical doping [2], irradiation [3], cold high-pressure densification (CHPD) [4] and hot isostatic pressure (HIP) [5] have so far been tried. Carbon doping has been proved to be one of the most efficient methods, since the C substitution for B in the MgB2 lattice will increase electron scattering, lower the
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electronic mean free path and hence improve the upper critical magnetic field (Bc2) and high-field Jc performance of MgB2 superconductors [6].
Until now, mainly three types of carbon source have been widely studied. (1) Nano-sized carbon source (SiC, C60 [7], carbon nanoparticles [8], nano-diamond [9],
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graphene [10], etc.). A significant improvement of high-field Jc and a slight reduction of low-field Jc in nano-SiC doped MgB2 has firstly been found by Prof. Dou’s group
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[11]. Sintering temperatures of ≧ 900 °C are needed for a better reaction in nano-carbon and carbon nanotube (CNT) doped MgB2 superconductors [12-14]. It
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was observed that nano-sized carbon sources are easy to agglomerate during the solid-solid mixing (SSM) with Mg and B powders due to the large surface energy. It
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is thus a great challenge to obtain homogeneous carbon distribution and substitution in nano-carbon source doped MgB2 superconductors [15].
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(2) Carbohydrates (sugar [16], malic acid [17-19], urea [20], glycerin [21],
glycine [22-24], etc.). In contrast to the nano-carbon source, carbohydrates are soluble in solvents so that the solution can mix with boron powders uniformly. Boron powders coated with carbohydrates or carbon can be obtained after heating the mixture of carbohydrate, boron powders and solvent at different temperatures [25, 26]. MgB2 with carbohydrates doping obtained by “wet” mixing showed a significant in-field Jc enhancement compared with that of the pure MgB2. However, there is so far almost no report comparing the doping effects of “wet” mixing and “dry” mixing 3
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(SSM method). (3) Hydrocarbon (benzene [27], ethyltoluene [28], pyrene [29], coronene [30], etc.). In contrast to carbohydrates, hydrocarbons have no oxygen, which will not introduce a large amount of MgO impurities into the MgB2 matrix. Maeda et al [31] discovered considerable in-field Jc improvement by pyrene (C16H10) gas diffusion into highly dense MgB2. Ye et al [32] used coronene (C24H12) as the carbon source. The 3
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nm coronene coating and 4 nm amorphous carbon coating were formed on nano-sized boron powders by heating the mixture of coronene and boron powders at 520 and 630 °C, respectively. The highest Jc value of 1.07×105 A/cm2 was achieved in coronene doped MgB2 wires at 4.2 K and 10 T [33]. In addition to hydrocarbons,
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some other carbon-containing materials may also be selected to process boron powders. Recently, a uniform coating of amorphous carbon on boron powders was obtained through the direct pyrolysis of PMMA polymer ((C5O2H8)n) at 500 and 800 °C [34].
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In the present study, low-cost malonic acid (C3H4O4) was first examined as a carbon source for improving the Jc performance of MgB2. We tried three methods of
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malonic acid addition: (1) “dry” mixing (SSM method), (2) “wet” mixing and vacuum drying at 60 °C (60VD method), and (3) “dry” mixing and sealed heat treatment at
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250 °C (250SH method). In addition, the decomposition behavior of malonic acid and the correlation between different doping amounts, doping methods, lattice parameters,
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microstructure and superconducting properties were investigated in detail.
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2. Experimental Mg
powders
(Alfa
Aesar,
325
mesh,
99.8%) ， amorphous
boron
(Pavezyum, >98.5%) and malonic acid (Alfa Aesar, 99.5%) were weighted in the ratio of MgB2 + y wt% (y = 0,5,10,15 and 30). All the powders were well mixed in an agate mortar and the mixture was pressed into pellets under the 3-ton uniaxial pressure. All the processes mentioned above were conducted in the glove box. The pellets were put inside the thick plastic bags and the bags were vacuumed and sealed using a sealing machine. The sealed plastic bags with pellets were put inside 4
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the oil of a hydraulic machine and then re-pressed under the 200 MPa isotropic pressures. The pellets were wrapped inside the Ta foils without sealing and then sintered at 800 °C for 3 hours in a tube furnace under Ar/H2 (4%) gas flow. This fabrication method was named as solid-solid mixing (SSM) method in the present work. The bulk MgB2 samples were ground into powders. The phase identification
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of the powder samples was performed by x-ray diffraction (Bruker D8 Advance). The PDXL2 software was used to calculate the full widths at half maximum (FWHM) values of the different diffraction peaks and the weight fraction of the MgO phase in the MgB2 matrix. The microstructures of the samples were examined by field
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emission scanning electron microscopy (FESEM). All the samples were cut to the size of a×b×c mm3 from as-sintered pellets. Zero field cooled (ZFC) magnetization versus temperature (M-T) and magnetization hysteresis (M-H) loops were obtained by using a quantum design vibrating sample magnetometer option (PPMS, Quantum Design).
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The Jc values were determined using the Bean model, Jc = 20ΔM/va(1−a/3b), where ΔM (emu) is the vertical width of the M-H loop, v is the volume of the measured
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sample, a and b are the cross-sectional sizes of the sample perpendicular to the applied field with b > a.
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According to the experimental results, among all the bulk MgB2 fabricated through SSM method, the 10 wt% malonic acid added MgB2 showed the highest Jc
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value. Hence, in order to analyze the effect of different doping methods on the superconducting performance of Mg(B1-xCx)2, the same amount of malonic acid
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addition (10 wt%) was used to pre-treat the boron powders. Two different methods were used. In study 1, the additive was dissolved in ethanol solution, mixed with boron powder, and then vacuum dried at 60 °C for 1 hour. The treated powder was mixed with Mg powder. The powder mixture was pressed and sintered under the same condition as that used for the SSM method. The obtained sample was named as 10 wt%-60VD (60VD for abbreviation). In study 2, the malonic acid was mixed with boron powder and pressed into pellets. The compressed mixtures were sealed in a vacuum silica tube and then heated at 250 °C for 1 hour. The pellets were then 5
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reground and mixed with Mg powder, followed by the same process used for SSM method. The obtained sample was named as 10 wt%-250SH (250SH for abbreviation). These samples were also characterized using the same methods as SSM samples.
3. Results and discussion 3.1 Thermal decomposition of the malonic acid
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It’s important to analyze the thermal decomposition process of malonic acid under high-purity argon atmosphere. Thermogravimetry (TG) and differential scanning calorimetry (DSC) analyzer NETZSCH STA 449C was used to measure the thermal properties of the malonic acid. The TG-DSC curve of malonic acid is shown in Fig. 1.
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A weight loss of 99 % is observed in the range of 120 to 200 °C. The endothermic peaks centered at 135 and 175 °C correspond to the melt and decomposition of malonic acid, respectively. A similar result was obtained in the TG-DTA curves of malonic acid measured by J. Györe et al. [35] The DTA peaks at 88 and 133 °C were
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recognized as the crystal structure modification and the melting point of malonic acid, respectively. In addition, the malonic acid decomposed at about 165°C according to
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the equation:
HOOC-CH2-COOH
CH3COOH + CO2
(1)
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There is no obvious weight loss between 200 and 650 °C in Fig. 1 since the decomposition products are all in the gaseous state. When the temperature exceeds
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440 °C, the acetic acid (CH3COOH) will start decomposing in two possible ways as
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follows [36]:
CH3COOH
CH4 + CO2
(2)
CH3COOH
CH2CO + H2O
(3)
The ketene (CH2CO) can decompose into C2H4 and CO. The C2H4 is likely to decompose to CH4 and C, but CH4 is very difficult to decompose into C and H2 below 1000 °C without the catalyst or high pressure [37]. It seems that the gaseous products of malonic acid cannot turn into nano-carbon efficiently. However, the gaseous products still affect the lattice structure and superconducting properties of MgB2, 6
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which will be discussed below.
Fig. 1. TG-DSC curve of malonic acid powder
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3.2 SSM method
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Fig. 2 shows the XRD patterns of MgB2 for different levels of malonic acid addition. All samples consist mainly of MgB2 and a less amount of MgO impurity.
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The relative intensity of the MgO peak increases monotonically with the adding amount of malonic acid. The same result can also be seen in Fig. 3(a) comparing the
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calculated weight fraction values of MgO phase in the MgB2 matrix. In particular, the MgO content of 30 wt% malonic acid added sample is about 3 times larger than that
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of the pure MgB2 sample. Since an isotropic pressure of 200 MPa was applied to the bulk mixture of Mg, B and malonic acid, part of oxygen-containing decomposition products of the malonic acid, including CO2, CO and H2O, may be sealed inside the bulk before the formation of the MgB2. It’s also very possible that the CO2 gas inside the compact bulk is not fully released out, because it can react with magnesium according to the reaction equation: 2Mg + CO2
2 MgO + C
(4)
This reaction was also confirmed in another carbohydrate - glycine doped MgB2 bulks 7
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by Cai et al through a thermodynamic and kinetic analysis of the MgO formation mechanism [24]. The carbon atoms generated from this reaction can replace B atoms in the formation process of MgB2. In addition, the H2O vapor may also contribute to
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the formation of the MgO.
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Fig. 2. XRD patterns of pure and malonic acid added MgB2 samples
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The FWHM values of the (100), (002) and (110) peaks for MgB2 samples with different amount of malonic acid addition are displayed in Fig. 3(b). The three peaks
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show a similar trend. The FWHM values first increase and then decrease with increasing amount of malonic acid additive. However, the peak broadening observed
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in XRD patterns is mainly related to the crystallite size and lattice strain except for the instrumental contribution. As demonstrated in many past studies, the broadening of the peaks in carbon source doped MgB2 could be explained by carbon doping induced grain size refinement [25] and by the lattice distortion (degradation of the crystallinity) originating from the C substitution for B [38]. Hence, the significant broadening of the peaks observed in malonic acid added MgB2 samples may also indicate smaller crystallite size or increased lattice distortion. 8
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MgO
a
8
6
4
2
0
5
10
15
20
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Weight fraction (%)
10
25
30
Amount of added malonic acid (wt %)
(100) (002) (110)
b
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0.6
0.4
0.3
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FWHM (deg.)
0.5
0.1
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0.2
0
5
10
15
20
25
30
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Amount of added malonic acid(wt%)
Fig. 3. (a) Weight fraction of MgO phase, (b) FWHM values of different diffraction peaks for the
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pure and different amount of malonic acid added MgB2 samples
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Scanning electron microscope (SEM) images of the pure and malonic acid added bulk MgB2 samples are presented in Fig. 4. The plate-shaped MgB2 grains are randomly distributed. The grains of the malonic acid added samples appear to be thinner and smaller than those of the pure MgB2. The grain size refinement is probably due to the impurity particles introduced by the doping, which serve as additional nucleation sites for grain formation. The additional impurity particles and, thus, nucleation sites can restrict the mobility of grain walls, preventing them from growth [25]. It can also be seen that the 15 wt% malonic acid added sample shows 9
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some cracks, 30 wt% added sample has large-sized pores, which will all result in poor connectivity. This indicates that overdoping induced large amount of gas releasing and MgO impurities will destroy the connectivity of MgB2.
(b)
5 μm
5 μm
(d)
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5 μm
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(e)
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(c)
5 μm
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(a)
5 μm
Fig. 4. SEM images of MgB2 + x wt% malonic acid samples with (a) x = 0; (b) x = 5; (c) x = 10; (d) x = 15; (e) x=30.
The lattice parameters a and c of MgB2 were calculated using the (110) and (002) peak positions, respectively. The actual level of C substitution in doped MgB2 was estimated through the relation: x = 7.5 × Δ(c/a) [39]. Fig. 5 shows the effect of the malonic acid addition on the structural parameters. As the malonic acid additions 10
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increase from 0 to 30 wt%, the a value decreases. In contrast, both c/a and x values of Mg(B1-xCx)2 increase. This result further confirms that C atoms produced by the reaction of Mg and CO2 indeed replaced the B atoms in Mg(B1-xCx)2 samples.
(a)
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a-axis (Å)
3.086
3.082 3.080 3.078
(b)
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c/a
1.145 1.144 1.143
0.01
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0.02
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x of C in Mg(B1-xCx)2
1.142
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10
20
30
Amount of added malonic acid (wt%)
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Fig. 5. (a) a-axis lattice parameter, (b) c/a value, and (c) actual carbon substitution level (x) in Mg(B1-xCx)2 for pure and different amount of malonic acid added MgB2 samples
The normalized zero field cooled (ZFC) magnetization versus temperature for the pure and malonic acid added bulk MgB2 are shown in Fig. 6. Compared with pure MgB2, a slight Tc decrease is observed for malonic acid added samples. Especially for 30 wt% added MgB2, the Tc value is about 37.03 K, which is 1.5 K lower than that of 11
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pure MgB2 due to carbon substitution induced impurity scattering. It is well known that grain connectivity, impurity and homogeneity in the MgB2 matrix will affect the superconducting transition width (ΔT). From M-T curves, it is seen that the ΔT values of MgB2 with 0, 5 and 10 wt% malonic acid are almost similar. However, the 15 wt% added sample shows much bigger ΔT and 30 wt% added MgB2 displays an incomplete superconducting transition curve. This result may be related to the larger
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amount of MgO impurity and the inferior crystallinity, which are both caused by malonic acid overdoping.
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H=20 Oe
-0.2
-0.4
-0.6
ZFC
-0.8
pure 5 wt% 10 wt% 15 wt% 30 wt%
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Normalized Magnetization
0.0
30
35
40
45
50
Temperature (K)
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25
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-1.0
Fig. 6. Temperature-dependent normalized magnetization for pure and malonic acid added MgB2
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samples, measured under zero field cooled condition.
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The field dependence of critical current density for the pure and malonic acid
added MgB2 at 20 K is observed in Fig. 7. The Jc values of 5 and 10 wt% added MgB2 are higher than that of the pure sample at all magnetic fields. The highest Jc value obtained by 10 wt% malonic acid added MgB2 is 5.3 times higher than pure MgB2 at 5 T. One reason for the Jc improvement is the enhanced grain boundary pinning caused by the refinement of grain size. In addition, carbon substitution for B sites induces higher Hc2 and contributes to the improvement of high-field Jc performance. For the 15 wt% malonic acid added MgB2, the low-field Jc is slightly depressed, but 12
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the high-field Jc is still much larger than that of the pure MgB2. Moreover, the 30 wt% added MgB2 shows the lowest Jc at low field, which may be due to a large amount of MgO impurity. However, the slope of the Jc - B curve for 30 wt% added MgB2 is the smallest among all samples, which should be due to the highest actual carbon substitution level (x) in Mg(B1-xCx)2.
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pure 5 wt% 10 wt% 15 wt% 30 wt%
5
4
10
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Jc(A/cm )
10
3
10
@ 20K 2
0
1
2
3
4
5
6
M
10
B (T)
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Fig. 7. Critical current density at 20 K vs. applied magnetic field for pure and malonic acid
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added MgB2 samples
The flux pinning force (Fp) is calculated by Fp= Jc × B and shown in Fig. 8(a) for
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pure and malonic acid added MgB2. The Fp values for the 10 wt% added MgB2 are much higher than for other samples in the entire magnetic field range. On the contrary,
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the low-field Fp values for 30 wt% added sample are much lower than those for pure MgB2. This may due to the poor connectivity caused by the large-sized pores and a large amount of MgO impurities. Furthermore, the normalized flux pinning force (Fp/Fp, max) versus the applied field for all samples is compared in Fig. 8(b). At high fields, the Fp/Fp, max values monotonically increase with the amount of malonic acid, which is consistent with the x variation in Mg(B1-xCx)2 shown in Fig. 5(c). This also confirms that carbon substitution for boron plays an important role in improving the high-field Jc performances. 13
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a
pure 5 wt% 10 wt% 15 wt% 30 wt%
3
Fp (N/cm )
1.0x10
8
0.0
0
1
2
3
4
6
pure 5 wt% 10 wt% 15 wt% 30 wt%
b
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1.0
0.5
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Fp / Fp,max
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B (T)
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3
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6
B (T)
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Fig. 8. Field dependence of the flux pinning force-Fp (a) and Fp/ Fp, max (b) for pure and malonic acid added MgB2 samples
3.3 60VD and 250SH methods From Table 1, it is seen that the 60VD and SSM samples show very similar lattice parameters, x and FWHM values. However, the weight fraction of the MgO for 60VD sample is a little higher than that for SSM sample. This may be explained by the introduced oxygen during the solution mixing of the malonic acid and nano boron powders in the air. The Fig. 9 shows that the Jc at 20 K, 5 T for the 60VD sample is 14
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7.2 times and 1.4 times higher than those for pure and malonic acid added samples prepared from the SSM method, respectively. It can be observed that the Jc of 60VD sample is higher than that of SSM sample in the entire magnetic field range. This is due to that the 60VD method can achieve a more uniform distribution of the malonic acid inside the boron powders and more homogeneous carbon doping [25]. In addition, the more uniformly distributed MgO and C impurity could contribute to limiting the
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growth of the MgB2 grains, resulting in larger grain boundary pinning. However, the Jc of SSM sample is only slightly lower than that of the 60VD sample. This may indicate that the gaseous carbon source such as CO2 could spread or diffuse within the
Sample ID
Lattice constant (Å)
Mg(B1-xCx)2
c/a
3.0863 1.1419
10 wt%-SSM
FWHM (deg.)
(100)
(002)
(110)
Weight fraction of MgO (%)
0
0.247
0.316
0.343
3.3
3.0811 1.1438
0.0143
0.295
0.341
0.422
4.6
10 wt%-60VD
3.0812 1.1438
0.0140
0.296
0.326
0.422
5.2
10 wt%-250SH
3.0839 1.1428
0.0067
0.270
0.328
0.376
3.75
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Pure MgB2
x value in
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a
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sample, so the SSM sample could also obtain effective doping effect.
Table 1. Lattice parameters, x values in Mg(B1-xCx)2, FWHM, weight fraction of MgO of pure and
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malonic acid added MgB2 samples using different doping methods.
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In order to verify the decomposition behavior of malonic acid, the compressed mixture of boron and malonic acid powders for 250SH sample was pre-treated at
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250 °C for 1 hour as mentioned before. This temperature was chosen to be higher than the decomposition temperature of malonic acid. As a result, all the structural parameters of the 250SH sample are between the values of pure MgB2 and those of SSM and 60VD samples as listed in Table 1. Besides, Fig. 9 shows that the high-field Jc performance of the 250SH sample is higher than for the pure MgB2 but lower than for SSM/60VD samples. These results demonstrated that the pre-heat treatment of compressed mixture at 250 °C followed by the grinding process for obtaining 15
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precursor powder at room temperature leads to a decrease of the actual carbon content in boron powders. This is because at least 1/3 of the C atoms contained in malonic acid can be released in the form of CO2 gas according to chemical reaction formula (1). However, a part of acetic acid still remained inside the boron powders after cooling down to room temperature.
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pure 10 wt%-SSM 10 wt%-60VD 10 wt%-250SH
5
2
Jc (A/cm )
10
4
3
10
@ 20K 2
0
2
M
10
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4
6
B (T)
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Fig. 9. The magnetic field dependence of Jc at 20K for pure and malonic acid added MgB2
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samples using different doping methods
It turns out that preparing the uniform nano-carbon coated boron through
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pre-heating the mixture of the malonic acid and amorphous boron powders at higher temperature is a complicated process. In fact, even after pyrolysis of malonic acid at
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1000 °C, approximately 3/4 of the carbon atoms will exist in the form of CO2 and CH4, and these gases will be released from the boron powders. Therefore, to prepare carbon-coated boron using malonic acid as the carbon source, a larger amount of malonic acid additions and appropriate reaction temperatures need to be further studied. On the other hand, malonic acid added MgB2 through SSM and 60VD methods obtained significant Jc improvement in the entire magnetic field range. In particular, the Jc of 60VD sample is 7.2 times higher than that of the pure MgB2 at 20 K and 5 T, thus indicating that malonic acid is an effective carbon source for 16
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enhancing the Jc of MgB2.
4. Conclusion In summary, bulk MgB2 samples with different amount of malonic acid addition were prepared using conventional solid state sintering at 800 °C. With increasing the amount of malonic acid, the lattice parameter a value decreased, while the MgO
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content in the MgB2 matrix increased. A suitable amount of MgO might be able to limit the growth of MgB2 grains. Furthermore, the 10 wt% malonic acid added MgB2 showed the highest Jc and Fp performance in the whole magnetic field range. The effect of three different doping methods on the structural and superconducting
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properties of MgB2 has been investigated. Interestingly, the Jc of SSM sample (made from “dry mixing”) was only slightly lower than 60VD sample (made from “wet mixing”), which could be attributed to the reaction of Mg and CO2 and the good diffusivity of CO2 within the sample.
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Acknowledgment
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This work is financially supported by the National Natural Science Foundation of China (51571002), by the General Program of Science and Technology Development
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Project of Beijing Municipal Education Commission of China (KM201810005010), by the Beijing Municipal Natural Science Foundation (2172008) and by 211 Program
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of Beijing City and Beijing University of Technology.
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