Study On Improving The Superconducting Properties Of Magnesium Diboride Bulks And Wires Through Organic Carbon Sources Doping

Since the superconductivity of magnesium diboride(MgB₂)was discovered in 2001,it has been widely concerned by researchers.Because the MgB₂ superconductor has a critical transition temperature(39 K)that is much higher than that of the low-temperature superconductor(LTS),and there is no weak link behavior of grain boundaries that is difficult to overcome in the high-temperature superconductor(HTS).Besides,the raw material cost is low and the fabrication processes are simple.Therefore,the MgB₂ superconductor has good application prospects due to its unique advantages.However,in order to meet its wide application requirements,it is still necessary to improve the upper critical field(H_irr),the irreversibility field(H_irr)and the high-field critical current density(J_c)of MgB₂.Chemical doping is a simple and effective way to improve the superconducting properties of MgB₂.Compared with other types of dopants such as metals and nano-carbons,organic carbon sources have achieved relatively better doping effects.In order to further improve the superconducting performance of MgB₂,it is necessary to try new organic carbon sources and explore the corresponding doping modification mechanism.The effects of different types of organic carbon sources and different organic carbon sources of the same type on the microstructure and superconducting properties of MgB₂bulks and wires were systematically studied in this paper.The corresponding doping mechanism was summarized through comparative analysis,which filled the gaps in current theoretical and experimental research.The important innovative research results obtained in this paper are as follows:For the first time,two different types of organic carbon sources,malonic acid(C₃H₄O₄)and hexamethylenediamine(C₆H₁₆N₂)were used for the doping modification of bulk MgB₂ prepared by in-situ solid state sintering.Through analyzing the thermal decomposition behavior and the doping effect of malonic acid and hexamethylenediamine,the corresponding doping mechanism was clarified.The results showed that the CO₂ produced by the decomposition of malonic acid reacted with Mg to release C and MgO.The C substituted the B in MgB₂ lattice,thus significantly improved the superconducting performances of MgB₂.However,when the doping amount is increased,a large amount of MgO impurities will be introduced,thereby reducing the superconducting performance of MgB₂.The optimal doping amount of malonic acid was 10 wt%.It was also found that unlike the doping mechanism of malonic acid,C substitution for B did not occur in hexamethylenediamine doped MgB₂,and no additional MgO impurities were introduced.At the same time,spherical MgB₂grains appeared inside the hexamethylenediamine doped samples,which reduced the average grain size of MgB₂and improved the connectivity of the grains,thereby effectively improving the J_c performance of MgB₂.The analysis results showed that the doping mechanisms of the two different types of carbon sources are different,and both have achieved excellent doping effects.Among them,at 20 K/5 T,the J_c values of 10wt%malonic acid-doped and 10 wt%hexamethylenediamine-doped MgB₂ were 7.2times and 5.6 times higher than that of the undoped sample,respectively.Different organic carbon sources of the same type were used for the doping modification of MgB₂ wire prepared by powder in tube(PIT)method.Three different polycyclic aromatic hydrocarbons(PAHs)including anthracene(C₁₄H₁₀),pentacene(C₂₂H₁₄)and coronene(C₂₄H₁₂)were selected as dopants.The effects of different PAHs on the microstructure and superconducting properties of the PIT-MgB₂ wires were systematically studied and compared.It was found that anthracene and pentacene which were used as new carbon sources for the first time have obtained a better doping effect than coronene.Besides,the doping mechanism of different PAHs was similar,that is,when heat treatment is performed at a low temperature for a short time,PAHs will hinder the reaction between Mg and B.The C released by the pyrolysis of PAHs substituted the B in the MgB₂ lattice.The J_c values at high field,the H_c2 and H_irrvalues in the low-temperature region were significantly enhanced through an appropriate amount of PAHs doping in PIT-MgB₂wires.Among them,the highest J_c value of1.8×10⁴A/cm² at 4.2 K/10 T was obtained by 3.75 at.%anthracene doped wire,which was 6 times higher than that of the undoped sample.In addition,the highest H_c2(0)value of 29 T was obtained by 5 at.%pentacene doped MgB₂ wire,which is 7.7 T higher than that of the undoped sample.In order to further improve the superconducting performance of MgB₂ wires,the above-mentioned PAHs were applied for the first time to MgB₂ wire prepared by IMD method.Besides,the effect of different PAHs doping on the microstructures and superconducting properties of the IMD-MgB₂ wires were systematically investigated.The results showed that there was a thick boron-rich layer near the Fe sheath in the undoped IMD wire.However,the homogeneous MgB₂ layers without the boron-rich layer were observed in PAHs doped samples.Combined with the analysis of microstructures,the doping mechanism of PAHs in IMD-MgB₂ wire was clarified that the doping of PAHs promoted the diffusion of liquid Mg to the B powder.The J_c values at high field,the H_c2 and H_irrvalues in the low-temperature region for PAHs doped IMD-wires were significantly improved through appropriately increasing the sintering temperature and prolonging the holding time.Among them,the highest J_c value of8.2×10⁴A/cm² at 4.2 K/10 T was obtained by 5 at.%anthracene doped wire,which is30 times higher than that of the undoped IMD wire and is 5 times higher than that of the anthracene doped PIT-MgB₂ wire.The highest H_c2(0)value of 33.5 T was obtained by 5 at.%anthracene doped IMD wire,which is 13.5 T higher than that of the pure IMD wire.In summary,this article is dedicated to improving the superconducting properties of MgB₂ bulk and wire and exploring the doping mechanism of different types of organic carbon sources.The effects of two different types of organic carbon sources,malonic acid and hexamethylenediamine,on the microstructure and superconducting properties of MgB₂ superconducting bulks were systematically studied.Based on this,different PAHs doped MgB₂ superconducting wires were prepared by the PIT method.The doping effects of different PAHs were systematically studied for the first time,and different PAHs were found to have similar doping modification mechanism.The analysis showed that the C released from the thermal decomposition of PAHs effectively replaced the B in the MgB₂ lattice,which is the main reason for the improved superconducting performance of MgB₂ superconducting wires.Finally,MgB₂superconducting wires doped with different PAHs were prepared by the IMD method.It was found that the doping of PAHs promoted the diffusion of Mg to the outermost B powder in the IMD wire,which reduced the boron-rich layer near the Fe sheath and formed a homogeneous MgB₂ layer.Besides,the superconducting performances of PAHs doped IMD-MgB₂ were much higher than of the pure IMD wire and PAHs doped PIT wires.In conclusion,by exploring new effective carbon sources,this article has significantly improved the superconducting properties of MgB₂ bulks and wires,discovered new doping mechanisms,and filled the gaps in current experimental and theoretical research related to doping modification of MgB₂.This has laid a certain experimental foundation for further promoting the application and basic research of MgB₂ superconducting materials.