Increased flux pinning force and critical current density in MgB2 by nano-La2O3 doping

MgB2 superconducting wires and bulks with nano-La2O3 addition have been studied. A series of MgB2 superconducting bulk samples with nano-La2O3 addition levels of 0, 5, 7, 18wt% were prepared. AC resistivity data showed slight increases of Bc2 and unchanged Birr for the bulk samples with doping levels lower than 7 wt% and decreased critical fields for the heavily doped (18 wt%) bulk. X-ray diffraction (XRD) showed the presence of LaB6 in the nano-La2O3 doped MgB2 bulk samples and decreased MgB2 grain size in nano-La2O3 doped bulks. Monocore powder-in-tube (PIT) MgB2 wires without and with 5 wt% nano-La2O3 addition (P-05) were prepared for transport property measurement. 2mol%C-doped Specialty Materials Inc. (SMI) boron powder was used for wire P-05 and previously prepared control wires (control wires were made without the addition of nano-La2O3 powder, W-00 and P2). Low field magnetic properties were obtained from magnetization loop (M–H), transport critical current density (Jc) was measured at 4.2 K for the nano-La2O3 doped PIT wire (P-05) and the control samples (P2 and W-00). The transport critical current density Jc (B) of P-05 at 4.2 K and 8 T (6.0 ×104 A/cm2) was twice that of the control wire. The critical magnetic fields (Bc2 and Birr) of P-05 and the control sample P2 were compared. The critical fields of P-05 were slightly less than those of P2. Kramer-Dew-Hughes plots indicated a change from surface pinning to a mixture of volume pinning and surface pinning. It is shown that enhancement of P-05’s transport properties is due to additional flux pinning by the fine-size rare-earth borides rather than enhanced Bc2 or Birr.


Introduction
Since the discovery of MgB 2 superconductors in 2001 [1], substantial improvement on the material has been achieved in terms of critical field, transport property, wire manufacture processes. Due to the relatively high T c (39 K) in MgB 2 and the shortage of liquid helium worldwide, MgB 2 is particularly useful for helium-free MgB 2 MRI magnets [2] Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd zhang.5952@osu.edu .

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Author manuscript and is expected to replace Nb-based MRI magnets in the future. To reach these goals, improvements in the critical current density (J c ) of MgB 2 conductors are necessary.
Numerous approaches have been taken to produce MgB 2 materials with high transport properties: (1) chemical doping of MgB 2 wires [3][4][5]; (2) cold pressing [6][7]; (3) hot isostatic pressing (HIP) [8][9]; (4) the introduction of the internal magnesium diffusion (IMD) method to address porosity and connectivity issues [10][11][12]. So far, the best "non barrier" transport J c values were obtained by the advanced internal magnesium infiltration (AIMI) approach, with the addition of C and Dy 2 O 3 (1.07 × 10 5 A/cm 2 ) at 10 T, 4.2 K, [13]. The AIMI technique has the benefit of forming dense MgB 2 layers with improved longitudinal connectivity compared to the extrinsic conventional powder-in-tube (PIT) method, which usually produces wires with randomly connected MgB 2 fibers. On the other hand, the PIT method is favored as a simple and inexpensive approach to studying wire properties in response to chemical doping.
Many chemicals have been added to MgB 2 in the past 18 years to study their effects on the resultant transport and other superconducting properties, such as upper critical field (Bc 2 ) [14][15], and irreversibility field (B irr ) [16][17]. Generally, improvements in J c have been attributed to the improvements of either Bc 2 or B irr . Till now, C has been shown to be the most effective doping element for the enhancement of Bc 2 [18] in MgB 2 materials. Many MgB 2 wires with high transport properties were doped with C or C-containing materials. In addition to C and C containing materials, doping with Dy 2 O 3 has shown to increase both J c and B irr [19][20]. In particular, Chen et al. [19] and Li et al. [20] both demonstrated the effectiveness of a combination of flux pinning by Dy 2 O 3 and carrier scattering by C. The effects on the transport J c of rare earth oxide additions such as Pr 6 O 11 [21][22], CeO 2 [23], [24] have also been studied. This paper describes the effect of adding nano-La 2 O 3 dopants on superconducting and structural properties in MgB 2 superconductors. La 2 O 3 was chosen because some rare-earth oxide additions have improved J c and Bc 2 /B irr [19][20][21][22][23][24] in MgB 2 superconductors. Besides, La 2 O 3 doping has been previously studied for MgB 2 tapes and nano LaB 6 flux pinning centers as well as increase of J c in the doped tapes were observed. Therefore, the effects of La 2 O 3 doping in MgB 2 wires and bulks were studied here to fully explore the doping effect as well as the optimum doping level. Consequently, four different doping levels (0, 5, 7,18 wt%) were chosen for the bulks and 5 wt% was chosen to make PIT wires based on the property measured for bulks with the same doping level. Transport J c in the 5 wt% nano-La 2 O 3 doped monocore PIT-processed MgB 2 wire increased to twice the value of the control sample at 4.2 K, 8 T. Nanomaterials Inc. The mole ratio of Mg to B powder used was 1: 2. The powder was mixed inside a glove box, and then transferred to a hydraulic press for densification at 5000 psi (or 34.5 MPa). The resulting pellets were heat treated at 850 °C for 30 minutes in flowing Ar, followed by furnace cooling.

Wire Samples.-
The wire samples were prepared by Hyper Tech Research using their well-known "continuous tube filling and forming" (CFTT) powder-in-tube (PIT) process [25] followed by wire drawing to 0.83 mm in diameter. Selected for controls were two samples from previous studies. Designated P2 [26] and W-00 [27] precursor powders were 2 mol% C doped B from SMI and Mg powder as in the undoped bulk. The sample specifications are listed in Table 1.

X-Ray diffraction (XRD) measurement.-
The powdered bulk samples were scanned on a Rigaku Miniflex 600 XRD machine at a scan rate of 5 deg / min. Phases and peaks were studied with the help of PDXL software.

AC resistivity.-
The AC resistivity vs temperature measurements on the bulk samples were performed on a Quantum Design Model 6000 Physical Property Measuring System (PPMS). The bulk samples were cut into rectangular prisms and four-point probe measurements were used to obtain sample resistance of 5 K to 45 K. The usual "10% normal resistivity" and "90% normal resistivity" rules were used to obtain B irr and Bc 2 , respectively.  [28], the 4.2 K magnetic J cm for the superconducting CTFF monocore strand and was derived from:

Transport
where d is the diameter of the MgB 2 layer inside the wire and ΔM is the full height of the M-H loop at a certain field. The magnetic J cm played a role in the pinning force calculation (see section 3.4).

Bulk sample's magnetic and structure properties
Bc 2 and B irr values were obtained for MgB 2 bulks doped with nano-La 2 O 3 and the control bulk sample (B-00) at temperatures of 16 K ~ 36 K, Figure 1. B irr stayed unchanged for B-05 and B-07. Bc 2 increased by 0.2 T with the addition of 5 wt% La 2 O 3 and increased by 0.4 T with addition of 7 wt% La 2 O 3 . However, both Bc 2 and B irr decreased significantly in response to doping with 18 wt% La 2 O 3 . Based on the data in Figure 1, it can be seen that T c of the doped bulks did not change compared to the control bulk sample. This is similar to the unchanged T c observation on Dy 2 O 3 doped MgB 2 bulks [29], meaning no significant atomic substitution occurred on MgB 2 host lattice sites.
Phases and peaks in the MgB 2 bulk samples have been studied with the aid of XRD. The patterns shown in Figure 2 indicates the presence of LaB 6 in all doped bulk samples (shown by green arrows), presumably there are pinning centers. The MgB 2 peaks of B-05 and B-07, which is consistent with the minor increases of Bc 2 seen in Figure 1. The lattice shifts are smaller than those observed by Gao et al. [30] in MgB 2 tapes with acetone and La 2 O 3 additions. Usually MgB 2 peak shifts observed in XRD patterns are caused by atomic substitution and/or strain. The shifts seen in Figure 3 is probably due to the lattice strain generated by the nanoparticles (such as LaB 6 ) since T c was shown to be unchanged in doped samples. MgB 2 grain size was estimated based on XRD data using William-Hall method, Table 1. As the La 2 O 3 doping level increases from 0 wt% to 7 wt%, MgB 2 grain size decreased from 20.6 nm to 7.6 nm. The grain size of MgB 2 was 17 nm in the heavily doped sample. Yuan [27] observed same trend in his Dy 2 O 3 doped MgB 2 bulks [27]. He attributed this grain size reduction to secondary phase DyB 6 nanoparticles. Here, the reduction of grain size in La 2 O 3 doped MgB 2 bulks might be due to the secondary phase LaB 6 , which inhibits the grain growth and pins fluxons.

Wire strength, transport and magnetic properties
This paper also describes the effect of 5 wt% nano-La 2 O 3 addition to MgB 2 PIT wires. Two previously-made control samples (P2 [26] and W-00 [27]) were used for reference. These two wires were interchangeably used not only because the compositions are the same and the heat treatment procedures are very similar, but also because measured data from two controls combined can offer complete data for comparison purpose in this paper. Though they were manufactured from different batches, these mono PIT wires (with the use of 2mol%C-doped B powder) usually possess consistent properties. The transport properties of PIT MgB 2 wire P-05 and undoped wires P2 and W-00 were measured at 4.2 K in transverse magnetic fields up to 12 T, Figure 3. It can be seen that La 2 O 3 doping resulted in a significant increase in J c . In particular, at 8 T and 4.2 K, the J c of P-05 is 6.0 ×10 4 A/cm 2 , about twice that of the control wire P2. It is important to keep in mind that AIMI approach usually produces MgB 2 wires with higher J c compared to conventional PIT method for wires with same composition and heat treatment procedure because of the enhanced connectivity. Thus, AIMI-produced wires are not discussed here.

Wire sample: Critical fields and flux pinning
In order to enquire into the factors responsible for the enhancement of J c , the critical fields (Bc 2 and B irr ) were measured via resistivity approach in the PPMS in Figure 4. The T c of P-05 decreased with the addition of La 2 O 3 and its Bc 2 decreased by about 1 T at lower temperatures and very insignificantly at higher temperatures. Likewise, its B irr decreased by around 0.6 T at lower temperatures and by a smaller amount at higher temperatures. A decrease in the Kramer field, B k (a surrogate for Birr) of 1.0 T at 4.2 K ( Figure 5) confirms the above result. The relatively larger Bc 2 values in the undoped wires could be due to the existence of nano-level electron scattering centers (e.g., grain boundaries, point defects, volume defects and secondary phases). Since these two wires shown on Figure 4 are manufactured at different heat treatment conditions, it is very likely that fine secondary impurity phases such as MgO, B 2 O 3 were introduced in the undoped wire P2 and therefore enhanced Bc 2 to a slight degree.
Two common mechanisms for enhancing J c in MgB 2 wires are to enhance B irr or enhance Bc 2 . Here, we propose another mechanism for enhanced J c in MgB 2 wires with nano-La 2 O 3 doping. It is well known that enhanced transport J c by way of C doping in MgB 2 wire can be attributed to enhanced B c2 by way of enhanced scattering effect [16,19]. The increased transport J c in Dy 2 O 3 -doped MgB 2 wire was attributed to an increase in B irr [19][20]. With regard to La 2 O 3 doped wire samples, as mentioned above, the 4.2 K, 8 T, J c of P-05 was about twice that of the undoped control wire. But this increase is accompanied by no changes or decreases in B c2 and B irr (1 T and 0.6 T, respectively, at lower temperatures), Figure 4. Clearly, a mechanism other than critical field is responsible for the increase in J c ; flux pinning is the obvious choice.
To further study the phenomena, Kramer plot has been made, A Kramer Plot, J c 0.5 B 0.25 vs.
B, was shown in Figure 5. The irreversibility fields based on Kramer model [31], B k , was taken at the x-axis intercepts of linear fittings (evenly spaced dashed lines) on Figure 5. B k of the doped wire has a lower value than the control wire value, which agrees with the critical field analysis done above on the wire samples.

Flux pinning in response to La 2 O 3 doping
Using data derived from the M-H loop, the normalized flux pinning force density F p /F p,max was plotted against normalized magnetic field b=B/B k at 20 K to avoid flux jump effect, Figure 6(a). B k here was derived based on magnetic measurement conducted at 20 K in PPMS. Figure 6(b) showed the J c data masured by transport and magnetic methods. Apparently, magnetic measured J c agrees well with the transport data for P-05. Based on Dew-Hughes's analysis [32] about f p ∝ b 1/2 (1 − b) 2 , the curve for the control sample which peaks at B/B k = 0.2 is indicative of grain boundary pinning. In contrast, the curve for P-05 wire peaked at around b = 0.168. This phenomenon has been observed in Dy 2 O 3 doped samples [29] and Yang et al. [29] stated that two reasons can be responsible for this behaviour: (1) The doped sample might contain a set of local B k s instead of one distinct value, which can lead to an artificial error in the estimation of the peak positions; (2) The deviation from b peak = 0.2 might be due to other pinning mechanisms (e.g., normal volume pinning in which f p maximizes at b < 0.2 due to anisotropy [33]) in association with the GB pinning. The presence of LaB 6 peaks in all the XRD patterns of the doped bulk samples strongly suggests that LaB 6 , formed by reaction with the B powder, would also be present in the doped wires and as such would be responsible for the observed volume pinning.

Concluding Discussion
Four MgB 2 bulk samples with 0-18 wt% nano-La 2 O 3 additions and a monocore PIT MgB 2 wire with 5 wt% nano-La 2 O 3 addition were prepared for measurements of J c , B irr , Bc 2 and magnetic and transport J c (see Table 1)    Upper critical field and irreversibility field for the control sample and nano-La 2 O 3 added PIT MgB 2 wire P-05.  (a, left) Normalized flux pinning force as a function of reduced field for control sample and the doped wire, P05, based on the magnetic data; (b, right) Critical current density measured for wire P-05 using both magnetic measurement and transport measurement. Both referred to as "Control"