**2. Accelerated sintering of MgB2 with different metal and alloy additions at low temperature**

According to the above sintering mechanism, the reaction between magnesium and boron at low temperature took a very long time to form the complete MgB2 phase as the result of the low diffusion rate of Mg atom at solid state. In order to rapidly synthesize the complete MgB2 phase through the low-temperature sintering, improving the diffusion efficiency of Mg atoms to the reaction interface is the key point, especially at the final stage of the sintering.

#### **2.1 The influence of different metals and alloys on the sintering process and superconductive properties of MgB2**

In order to accelerate the diffusion rate of Mg atoms and thus improve sintering efficiency of MgB2 at low temperature, different metals and alloys dopants were added into Mg-2B sintering system by world-wide research groups.

work is also comparable to the calculated value in Shi et al.'s study [19] whereas the models are different from theirs. But in their study, the activation energy is calculated using a

Fig. 8. Schematic illustration of solid-solid reaction between Mg and B particles based on the

Based on above discussion, it is concluded that the reaction between Mg and B during the low-temperature sintering is controlled by varied mechanisms. At initial stage, the reaction rate is mainly determined by the phase boundary reaction mechanism. As the reaction prolonging and the synthesized MgB2 layer increasing, the diffusion-limited mechanism gradually becomes dominant. The corresponding activation energy is also decreased firstly

**2. Accelerated sintering of MgB2 with different metal and alloy additions at** 

**2.1 The influence of different metals and alloys on the sintering process and** 

According to the above sintering mechanism, the reaction between magnesium and boron at low temperature took a very long time to form the complete MgB2 phase as the result of the low diffusion rate of Mg atom at solid state. In order to rapidly synthesize the complete MgB2 phase through the low-temperature sintering, improving the diffusion efficiency of Mg atoms to the reaction interface is the key point, especially at the final stage of the

In order to accelerate the diffusion rate of Mg atoms and thus improve sintering efficiency of MgB2 at low temperature, different metals and alloys dopants were added into Mg-2B

model-free method, just as in our work.

inter-diffusion mechanism [39].

and then increased again.

**superconductive properties of MgB2** 

sintering system by world-wide research groups.

**low temperature** 

sintering.

Shimoyama *et al.* [40] found that a small amount of silver addition decreases dramatically the reaction temperature of magnesium and boron in the formation of bulk MgB2 without degradation of either the critical temperature or the critical current density. Although the added silver forms an Ag-Mg alloy after the heat treatment, these impurity particles exist mainly at the edge of voids in the sample microstructure and therefore do not provide a significant additional restriction to the effective current path. Accordingly MgB2 bulks with excellent *J*c properties have been fabricated at a temperature as low as 550 °C with the 3 at.% Ag doping. The sintering time of doped samples is also reduced significantly compared to that required for undoped samples fabricated by low temperature sintering. This effectively widens the processing window for the development of practical, low-cost MgB2 superconductors by reaction at low temperature [40].

Hishinuma *et al.* [41] synthesized Mg2Cu-doped MgB2 wires with improved *J*c by sintering at low temperature for 10 h. They found that the formation of the MgB2 phase is improved due directly to the lower melting point of Mg2Cu (568 °C) than Mg (650 °C), which can promote the diffusion of Mg in the partial liquid (the MgB2 phase forms by the diffusion reaction between released Mg from Mg2Cu and amorphous B powder [41]). The *J*c of sample prepared in this way can be improved further in Mg2Cu-doped MgB2 wires by sintering at lower temperature (450 °C) for longer time (more than 100 h). The maximum core *J*<sup>c</sup> value of these samples was found to be over 100, 000 A cm-2 at 4.2 K in a magnetic field of 5 T for a tape sintered for 200 h. Bulk MgB2 has been fabricated successfully in other studies by Cudoping and sintering at 575 oC for only 5 h [42]. Thermal analysis indicates that the Mg-Cu liquid forms through the eutectic reaction between Mg and Cu at about 485 °C, which leads to the accelerated formation of MgB2 phase at low temperature. The SEM images of the sintered Cu-doped samples are shown in Fig. 9. It is observed that the undoped sample is porous and consists of small irregular MgB2 particles and large regular Mg particles which are in poor connection with each other (see Fig. 9a). On the other hand, the MgB2 particles of the doped sample become larger and more regular accompanying with the increasing amount of Cu addition. The doped samples also become denser with the amount of Cu addition increasing for the reason that the MgB2 particles are in better connection with each other and give birth to less voids (see Figs. 9b-9d). Especially, as shown in the Fig. 9d, the MgB2 grains in the (Mg1.1B2)0.9Cu0.1 sample exhibit platy structure with a typical hexagonal shape [42]. The high *J*c in MgB2 samples doped with Cu is attributed mainly to the grain boundary pinning mechanism that results from the formation of small MgB2 grains during low temperature sintering. As with the Ag-doped samples, the concentration of Mg-Cu alloy in these samples tends to form at the edge of voids in the microstructure and does not degrade significantly the connectivity between MgB2 grains, which contributes directly to enhanced *J*c. Recently, the addition of Sn to the precursor powder has also been observed to assist the formation of the MgB2 phase during low temperature sintering, and bulk Sndoped MgB2 prepared at 600 ºC for 5 h exhibit good values of *J*c [43].

Interestingly, although Ag and Sn addition can form a local eutectic liquid with the Mg precursor at lower temperature than the addition of Cu, the Cu has been found to play a more effective role in the improvement of MgB2 phase formation than Ag and Sn at low temperature. The Cu-doped samples take significantly less time to form the primary MgB2 phase than those containing similar concentrations of Ag or Sn at a similar sintering

Sintering Process and Its Mechanism of MgB2 Superconductors 483

The addition of minor metals or metal alloys additions represents the most convenient, effective and inexpensive way of preparing MgB2 with excellent superconducting properties at low sintering temperatures. As a result, the accelerated sintering mechanism apparent in the effective processing of these samples should be clarified. The accelerated sintering mechanism of precursors containing Cu, for example, has been studied in detail using

As discussed in section 1.1, thermal analysis of the sintering process of undoped MgB2 reveals three peaks corresponding to solid-solid reaction, Mg melting and solid-liquid reaction (see Fig. 10). A similar process was observed in samples of composition (Mg1.1B2)1 xCux with x = 0.01, 0.03, 0.05 and 0.10, except that the on-set temperature of each the three peaks decreased gradually with increasing amount of Cu addition. It should be noted that an apparent endothermic peak appears at about 485 ºC, which is just below the first exothermic peak in the thermal analysis curves of the (Mg1.1B2)0.90Cu0.10 samples. By reference to the binary Mg-Cu phase diagram (see Fig. 11), it is apparent that the Mg-Cu liquid initially forms locally through the eutectic reaction during the sintering process of Mg-Cu-B system, resulting in the appearance of this apparent endothermic peak. The local formation of Mg-Cu liquid in the (Mg1.1B2)1-xCux samples with x = 0.01, 0.03 and 0.05 is limited for the small amount of Cu added to the precursor, which results in an endothermic signal that is too small to be detected by the thermal analysis measurement in this

Fig. 10. Measured thermal analysis curves during the sintering of (Mg1.1B2)1-xCux (with x = 0,

0.01, 0.03, 0.05 and 0.10) samples with an applied heating rate of 20 ºC /min [46].

**2.2 The mechanism of metal-accelerated sintering at low temperature** 

thermal analysis and activated sintering theory [44, 45, 46].

temperature range [46].

temperature [40, 42, 43]. Grivel *et al.* [44] observed a similar phenomenon in a study of the effects of both Cu and Ag on the kinetics of MgB2 phase formation. The addition of 3 at.% Cu or Ag to a precursor mixture consisting of Mg and B powders results in a significant increase of MgB2 phase formation kinetics in the temperature range below the melting point of Mg. The MgB2 phase forms more rapidly in the precursors containing Cu than those containing Ag. These authors suggest that this behavior might be related to the lower solubility limit of Cu in solid Mg, compared to the case of the Mg-Ag system [44].

Fig. 9. SEM images of the microstructures of the (Mg1.1B2)1-xCux samples sintered at 575 oC for 5 h with (a) x = 0.0, (b) x = 0.03, (c) x = 0.05 and (d) x = 0.10, respectively [42].

Based on the above discussion, the assisted sintering of MgB2 with different metal or metal alloy additions at low temperature is convenient from a practical processing point of view and also reduces the processing time. In addition, these additives tend to be cheap and yield MgB2 samples with improved *J*c. Therefore, this technique appears to be the most promising way of preparing practical, low-cost MgB2 superconductors at low temperature, compared to the use of different Mg-based precursors and the ball milling pretreatment of precursor powders.

temperature [40, 42, 43]. Grivel *et al.* [44] observed a similar phenomenon in a study of the effects of both Cu and Ag on the kinetics of MgB2 phase formation. The addition of 3 at.% Cu or Ag to a precursor mixture consisting of Mg and B powders results in a significant increase of MgB2 phase formation kinetics in the temperature range below the melting point of Mg. The MgB2 phase forms more rapidly in the precursors containing Cu than those containing Ag. These authors suggest that this behavior might be related to the lower

solubility limit of Cu in solid Mg, compared to the case of the Mg-Ag system [44].

(a) (b)

(c) (d)

Fig. 9. SEM images of the microstructures of the (Mg1.1B2)1-xCux samples sintered at 575 oC

Based on the above discussion, the assisted sintering of MgB2 with different metal or metal alloy additions at low temperature is convenient from a practical processing point of view and also reduces the processing time. In addition, these additives tend to be cheap and yield MgB2 samples with improved *J*c. Therefore, this technique appears to be the most promising way of preparing practical, low-cost MgB2 superconductors at low temperature, compared to the use of different Mg-based precursors and the ball milling pretreatment of precursor powders.

for 5 h with (a) x = 0.0, (b) x = 0.03, (c) x = 0.05 and (d) x = 0.10, respectively [42].

#### **2.2 The mechanism of metal-accelerated sintering at low temperature**

The addition of minor metals or metal alloys additions represents the most convenient, effective and inexpensive way of preparing MgB2 with excellent superconducting properties at low sintering temperatures. As a result, the accelerated sintering mechanism apparent in the effective processing of these samples should be clarified. The accelerated sintering mechanism of precursors containing Cu, for example, has been studied in detail using thermal analysis and activated sintering theory [44, 45, 46].

As discussed in section 1.1, thermal analysis of the sintering process of undoped MgB2 reveals three peaks corresponding to solid-solid reaction, Mg melting and solid-liquid reaction (see Fig. 10). A similar process was observed in samples of composition (Mg1.1B2)1 xCux with x = 0.01, 0.03, 0.05 and 0.10, except that the on-set temperature of each the three peaks decreased gradually with increasing amount of Cu addition. It should be noted that an apparent endothermic peak appears at about 485 ºC, which is just below the first exothermic peak in the thermal analysis curves of the (Mg1.1B2)0.90Cu0.10 samples. By reference to the binary Mg-Cu phase diagram (see Fig. 11), it is apparent that the Mg-Cu liquid initially forms locally through the eutectic reaction during the sintering process of Mg-Cu-B system, resulting in the appearance of this apparent endothermic peak. The local formation of Mg-Cu liquid in the (Mg1.1B2)1-xCux samples with x = 0.01, 0.03 and 0.05 is limited for the small amount of Cu added to the precursor, which results in an endothermic signal that is too small to be detected by the thermal analysis measurement in this temperature range [46].

Fig. 10. Measured thermal analysis curves during the sintering of (Mg1.1B2)1-xCux (with x = 0, 0.01, 0.03, 0.05 and 0.10) samples with an applied heating rate of 20 ºC /min [46].

Sintering Process and Its Mechanism of MgB2 Superconductors 485

MgB2 phase at low temperature, even though the latter can form liquid at much lower temperature. Inspection of the relevant binary phase diagram of Sn and Ag with Mg (not shown here) indicates that the solubility limit of Mg in Sn is much lower than that in Cu. Hence, it is more difficult for the Sn-based local liquid to wet the Mg particles and promote the diffusion of Mg according to the *Solubility* criterion. As a result, the activated sintering of MgB2 with Sn addition is much less efficient than Cu addition. On the other hand, the solubility limit of Mg in Ag is higher than that in Cu and the Ag-based local liquid should wet the Mg particles more easily and accelerate the diffusion of Mg more effectively. However, the solubility limit of Ag in solid Mg is also much higher than for the case of Cu, which means that the amount of local Ag-based liquid present will decrease due to the solution of Ag in the Mg solid. The effect of this is to lower the activated sintering efficiency

compared to that obtained for a similar concentration of Cu addition.

Fig. 12. An ideal phase diagram for the activated sintering system [45].

Fig. 11. Binary Mg-Cu phase diagram [48].

This raises the question of how the presence of a local Mg-Cu liquid accelerates the subsequent formation of MgB2 phase. (in previous studies [45, 46], the accelerated sintering mechanism of MgB2 with Cu addition is attributed to the activated sintering).

It is known that activated sintering of MgB2 by chemical doping facilitates either a lower sintering temperature or a shorter sintering time. German *et al.* [45] have proposed the three criteria for activated sintering systems of solubility, segregation and diffusion as follows:


Accordingly, an ideal phase diagram for the activated sintering system can be constructed (see Fig. 12). The formation of the MgB2 phase in the Mg-Cu-B system, is controlled mainly by the diffusion rate of Mg atoms. Only the effect of Cu addition on this diffusion rate is considered to be significant, and the effect of Cu addition on the B atoms can be neglected. Hence, the Mg-Cu-B system can be simplified as an Mg-Cu system for the analysis of the influence of Cu addition on the sintering process. As shown in the Mg-Cu phase diagram (see Fig. 11), Cu addition dramatically decreases the liquidus and the solidus, which implies that the local Mg-Cu liquid can segregate to the interface of Mg particles and therefore meets the *Segregation* criterion in German's study [45]. The high solubility of Cu for Mg (see Figs 11 and 12) enables the Mg-Cu liquid to wet the Mg particles and support the diffusion transport mechanism. As a result, this meets the *solubility* criterion. Generally, the atomic diffusivity in the liquid state is larger than that in the solid state, so the Mg-Cu liquid also meets the *Diffusion* criterion. Collectively, these observations suggest theoretically that the local Mg-Cu liquid meets all the criteria for the diffusion of Mg to B. Cu, therefore, can serve as the activated sintering addition and accelerate the formation of the MgB2 phase. The conclusion can also be verified by the microstructure observation of the Cu-doped sample sintered at low temperature, as shown in Fig. 13 [46]. It was clear that Cu was concentrated at local region by the edge of voids while Mg was preferentially distributed inside of particles far away from the voids. The result indicated that the Mg-Cu alloys corresponding to the local Cu-Mg liquid during the sintering process mainly concentrated at the edge of voids. Since the void results from the diffusion of Mg atom into B during sintering as mentioned previously [21], the concentration of Mg-Cu alloys at the edge of the voids implied that the Mg-Cu liquid generated and segregated to the interface between Mg particles and B particles at the initial stage of the sintering process and then provided a high transport for the diffusion of Mg into B. After a period of sintering time, Mg was run out and voids formed at the former place of the Mg.

Other metals or metal alloys must first form local liquids with Mg before the formation of MgB2 phase if they are to serve as activate additives during low temperature sintering. Whether or not these local liquids promote the formation of the MgB2 phase and activate the sintering mechanism, should be verified by considering the criteria described above (for the case of Cu addition), the ideal phase diagram for the activated sintering system and the binary phase diagram of the added element and Mg. In the section 2.1, the addition of Cu was demonstrated to be more effective than Sn or Ag in accelerating the formation of the

This raises the question of how the presence of a local Mg-Cu liquid accelerates the subsequent formation of MgB2 phase. (in previous studies [45, 46], the accelerated sintering

It is known that activated sintering of MgB2 by chemical doping facilitates either a lower sintering temperature or a shorter sintering time. German *et al.* [45] have proposed the three criteria for activated sintering systems of solubility, segregation and diffusion as follows: i. *Solubility* The additive A must have a high solubility for the base B, while the base B must have a low solubility for the additive A so that the additive can wet the base

ii. *Segregation* During the sintering, the additive must remain segregated at the inter-

iii. *Diffusion* The diffusivity of the base metal B in the additive layer must be higher than

Accordingly, an ideal phase diagram for the activated sintering system can be constructed (see Fig. 12). The formation of the MgB2 phase in the Mg-Cu-B system, is controlled mainly by the diffusion rate of Mg atoms. Only the effect of Cu addition on this diffusion rate is considered to be significant, and the effect of Cu addition on the B atoms can be neglected. Hence, the Mg-Cu-B system can be simplified as an Mg-Cu system for the analysis of the influence of Cu addition on the sintering process. As shown in the Mg-Cu phase diagram (see Fig. 11), Cu addition dramatically decreases the liquidus and the solidus, which implies that the local Mg-Cu liquid can segregate to the interface of Mg particles and therefore meets the *Segregation* criterion in German's study [45]. The high solubility of Cu for Mg (see Figs 11 and 12) enables the Mg-Cu liquid to wet the Mg particles and support the diffusion transport mechanism. As a result, this meets the *solubility* criterion. Generally, the atomic diffusivity in the liquid state is larger than that in the solid state, so the Mg-Cu liquid also meets the *Diffusion* criterion. Collectively, these observations suggest theoretically that the local Mg-Cu liquid meets all the criteria for the diffusion of Mg to B. Cu, therefore, can serve as the activated sintering addition and accelerate the formation of the MgB2 phase. The conclusion can also be verified by the microstructure observation of the Cu-doped sample sintered at low temperature, as shown in Fig. 13 [46]. It was clear that Cu was concentrated at local region by the edge of voids while Mg was preferentially distributed inside of particles far away from the voids. The result indicated that the Mg-Cu alloys corresponding to the local Cu-Mg liquid during the sintering process mainly concentrated at the edge of voids. Since the void results from the diffusion of Mg atom into B during sintering as mentioned previously [21], the concentration of Mg-Cu alloys at the edge of the voids implied that the Mg-Cu liquid generated and segregated to the interface between Mg particles and B particles at the initial stage of the sintering process and then provided a high transport for the diffusion of Mg into B. After a period of sintering time,

Other metals or metal alloys must first form local liquids with Mg before the formation of MgB2 phase if they are to serve as activate additives during low temperature sintering. Whether or not these local liquids promote the formation of the MgB2 phase and activate the sintering mechanism, should be verified by considering the criteria described above (for the case of Cu addition), the ideal phase diagram for the activated sintering system and the binary phase diagram of the added element and Mg. In the section 2.1, the addition of Cu was demonstrated to be more effective than Sn or Ag in accelerating the formation of the

particle interfaces to remain effective during the entire sintering process.

mechanism of MgB2 with Cu addition is attributed to the activated sintering).

particles and then exhibit a favorable effect on diffusion.

Mg was run out and voids formed at the former place of the Mg.

the self-diffusivity of the base metal B.

MgB2 phase at low temperature, even though the latter can form liquid at much lower temperature. Inspection of the relevant binary phase diagram of Sn and Ag with Mg (not shown here) indicates that the solubility limit of Mg in Sn is much lower than that in Cu. Hence, it is more difficult for the Sn-based local liquid to wet the Mg particles and promote the diffusion of Mg according to the *Solubility* criterion. As a result, the activated sintering of MgB2 with Sn addition is much less efficient than Cu addition. On the other hand, the solubility limit of Mg in Ag is higher than that in Cu and the Ag-based local liquid should wet the Mg particles more easily and accelerate the diffusion of Mg more effectively. However, the solubility limit of Ag in solid Mg is also much higher than for the case of Cu, which means that the amount of local Ag-based liquid present will decrease due to the solution of Ag in the Mg solid. The effect of this is to lower the activated sintering efficiency compared to that obtained for a similar concentration of Cu addition.

Fig. 11. Binary Mg-Cu phase diagram [48].

Fig. 12. An ideal phase diagram for the activated sintering system [45].

Sintering Process and Its Mechanism of MgB2 Superconductors 487

In addition, following these criteria and inspection of the appropriate binary phase diagrams of the additive metal elements with Mg, effective activated addition for the low temperature sintering of MgB2 could be achieved potentially using lots of candidate metals. Accordingly, Cu was finally determined as effective activator for improving sintering

**3. The effect of Cu activator on the microstructure and superconductive** 

Cu addition can improve the sintering efficiency of MgB2 and thus selected as sintering activator. However, whether Cu activator optimizes the microstructure and superconductive properties of MgB2? To answer this question, the effect of Cu activator on the microstructure of MgB2 sintered at both low temperature and high temperature were

**3.1 Effect of Cu activator on the reduction of MgO impurity in MgB2 sintered at high** 

MgO is always present as the inevitable impurity phase during the sintering process of MgB2 for the reason that Mg is very reactive with oxygen, which can be supplied by the gaseous O impurity in the protective Ar gas and the oxide impurity (such as B2O3 impurity in the B powders) in the starting materials. The presence of MgO impurity may be of great importance and yields a significant effect on the superconductive properties of MgB2 superconductor. Although the MgO nanoinclusions within MgB2 grains could serve as strong flux pinning centers when their size were comparable to the coherent length of MgB2 (.approximately 6~7 nm), the presence of excess MgO phases or largesized MgO particles at the grain boundaries could result in the degradation of grain connectivity [49, 50]. Hence, it is essentially important to control the amount of MgO impurity during the sintering of MgB2

In our previous work [51], based on the investigation of the effect of minor Cu addition on the phase formation of MgB2, it is found that the minor Cu addition (<3 at %) could apparently reduce the amount of MgO impurity in the prepared MgB2 samples, which provided a new route to govern the oxidation of Mg during the in-situ sintering of MgB2

Figure 14 shows the X-ray diffraction patterns of the (Mg1.1B2)1-xCux (x = 0.0, 0.01, 0.03 and 0.10) samples sintered at 850oC for 30min. It can be seen that all the sintered samples contain MgB2 as the main phase. In the undoped samples, the MgO peaks are easily recognized, which suggests that some Mg was oxidized during the sintering process and thus MgO was the main impurity in the sintered samples. On the other hand, in the diffracted patterns of the Cu-doped samples, all the MgO phase peaks become weaker and even some peaks identified as MgO phase disappear with the amount of Cu addition increasing. This trend can be observed more clearly from the Fig. 15, which shows the most intense peak (the peak of (200) crystal plane) of MgO in the X-ray diffraction patterns of the sintered (Mg1.1B2)1-xCux (x = 0.0, 0.01 and 0.03) samples. The results suggest that the minor Cu addition can depress

the oxidation of Mg apparently during the in-situ sintering of MgB2 samples.

efficiency of MgB2 in our work.

investigated in detail.

**temperature** 

samples.

**properties of MgB2 prepared by sintering** 

samples by altering the amount of Cu addition.

Fig. 13. A secondary electron image and elemental maps of Cu and Mg for (Mg1.1B2)0.8Cu0.2 sample sintered at 575 oC for 5 h [46].

Fig. 13. A secondary electron image and elemental maps of Cu and Mg for (Mg1.1B2)0.8Cu0.2

sample sintered at 575 oC for 5 h [46].

In addition, following these criteria and inspection of the appropriate binary phase diagrams of the additive metal elements with Mg, effective activated addition for the low temperature sintering of MgB2 could be achieved potentially using lots of candidate metals. Accordingly, Cu was finally determined as effective activator for improving sintering efficiency of MgB2 in our work.
