二硼化镁的形成机理及其成相控制
摘要
随着人们对二硼化镁(MgB2)超导材料研究的不断深入,关于其形成机理及成相控制的研究对高品质MgB2超导材料的制备具有越来越重要的指导意义。本文采用传统的固相烧结和高频感应熔炼的方法分别制备MgB2多晶块体材料及Mg-Cu-B三元合金,并结合显微组织观察、差热分析技术、粉末烧结理论,热力学和动力学分析手段,系统研究了原位烧结过程及深过冷Mg-Cu-B合金熔体中MgB2相的形成规律。另外,在原位烧结过程中,通过对外加和自生纳米MgO颗粒掺杂相自身演化规律和对MgB2相形成过程影响的研究,确定了这些影响与MgB2超导电性之间的必然联系。上述研究包含的主要内容和获得的结论有:
利用高精度差热分析仪对Mg粉和B粉的混合试样进行烧结处理并同步监控其反应进程,在实验分析的基础上,结合经典烧结理论和多重扫描动力学分析方法,系统研究了原位烧结制备过程中多晶MgB2相的形成过程及成相机理,并确定了最佳烧结工艺。结果表明:Mg粉和B粉在Mg熔化之前便已开始反应,直到1023K时才反应完全并生成多晶MgB2相,因此在略高于1023K烧结得到的样品具有高的超导转变温度。在Mg粉熔化之前,反应属于固-固反应阶段,相互接触的Mg和B颗粒通过原子之间的相互扩散在接触处形成固溶活化区,MgB2相便在该活化区内形成。多重扫描动力学分析结果显示:固-固反应所对应的最概然机理函数为Avrami-Erofeev,表明MgB2相在该阶段受随机形核和随后的瞬时生长方式所控制,生成的MgB2颗粒以纳米尺度存在且很难长大;Mg粉熔化以后,反应进入固-液反应阶段,熔融Mg的渗透和包覆作用促进了Mg和残余B之间的反应,与此同时,MgB2晶粒在该阶段按Ostwald熟化机制迅速长成规则的六边形。
在上述工作基础上,同样采用热分析的手段,进一步研究了原位烧结过程中纳米Al2O3粉末掺杂对多晶MgB2相形成过程的影响,并确定了该影响与超导电性之间的内在联系。结果表明:在Mg粉和B粉原位烧结反应过程中,纳米Al2O3将与Mg发生反应而生成纳米MgO和单质Al,由此改变了MgB2相的形成进程。纳米MgO颗粒的析出限制了MgB2晶粒的长大,从而有效增加了MgB2多晶体的晶界磁通钉扎作用。同时,纳米MgO颗粒自身也可作为有效的磁通钉扎中心,提高MgB2多晶体的临界电流密度。但随着纳米Al2O3粉末掺杂量的增多,析出的纳米MgO颗粒将发生局部团聚和长大而失去磁通钉扎作用,从而破坏了MgB2多晶体的整体超导电性。Al原子部分取代MgB2晶体结构中Mg原子的位置,从而破坏了它的电子结构而显著降低MgB2多晶体的临界转变温度。
在原位烧结制备多晶MgB2超导体过程中,MgO作为第二相的析出不可避免。对自生MgO相的形成、演化规律及对超导电性能影响的研究发现:通过调整固-固反应阶段的保温时间可实现自生纳米MgO颗粒的控制析出,并将其保留到固-液反应阶段的大角度MgB2晶界及表面处形成有效的磁通钉扎中心,从而得到高超导电性能的多晶MgB2超导体。
选取共晶Mg58.0Cu42.0和Cu86.7B13.3作为Mg和B的先驱相,采用高频感应熔炼的方法在常压及较低温度(1100℃)条件下熔配Mg15Cu75B10三元合金。随后,利用循环过热的净化方法得到不同初始过冷度条件下的深过冷Mg15Cu75B10合金,并借助经典形核理论和瞬态形核理论,讨论了Mg15Cu75B10合金组织中MgB2和MgB4相之间竞争形核的热力学和动力学条件,给出了过冷Mg15Cu75B10合金组织中形成MgB2相的临界条件。结果表明:在小过冷度范围内,当过冷度大于某一临界过冷度时,MgB2相比MgB4相具有更低的形核功、更大的形核率和更短的形核孕育时间,因而MgB2相从与MgB4相的竞争形核中胜出,优先从合金熔体中形核并生长。然而,在大的初始过冷度(大于223K)下,MgB2相和MgB4相均具有较小的形核率和较长的形核孕育时间,很难从过冷熔体中形核。
The researches on the formation mechanism and controlling of magnesium diboride (MgB2) have become more and more important for guiding the preparation of high quality MgB2 superconductors by optimzing the prepration process. In the present paper, polycrystalline MgB2 superconductor and Mg-Cu-B ternay alloy were prepared by traditional solid-state sintering method and high frequency induction metling method respectively. Moreover, the evolution mechanisms of MgB2 phase during the in-situ sintering process and in the undercooed Mg-Cu-B alloy melt were systematically explored by means of metallographic analysis, diffrerential thermal analysis, powders sintering theory, thermodynamics and kinetics ananlysis. Additionally, the evolutions of ex-situ and in-situ formed nano-dopants during in-situ sintering process, as well as, the effects on the formation process and superconductivity of polycrystalline MgB2 were clarified. More details were given as follows.
A high-resolution Differerntial Thermal Analysis (DTA) apparatus was used to sinter the mixture of Mg and B powder and monitor the reaction process. Based on the classical sintering theory and multiple scan kinetic analysis method, the formation process and mechanism of polycrystalline MgB2 during in-situ sintering process were systemticlly investigated and the optimal sintering process was determined. It indicates that the reaction between Mg and B powders starts before Mg melting and not complete until the sintering temperature exceeds to 1023K, which is the best temperature to obtain MgB2 superconductor exhibiting the highest critical transition temperature (Tc). Solid-solid reaction stage (before Mg melting), the solution activated regions formed ahead at the contact areas between Mg and B particles by diffusion of atoms, then MgB2 phase precipitated from these regions. Furthermore, multiple scan kinetic analysis shows that the most probable mechanism function of this solid-solid reaction is Avrami–Erofeev equation, which represents that the formation of MgB2 phase is controlled by a mode of andom nucleation followed by instantaneous growth of nuclei. Therefore, the grains are in nano-scale at this stage. At the solid-liquid reaction stage (after Mg melting), the molten Mg promotes the reaction between Mg and residual B powders under the infilteration and enwrapping effects. Finally, the formed MgB2 grains grow up to regular hexagon morphology rapidly by Ostwald ripening mechanism.
Based on the above observations, the doping effect of nano-Al2O3 powders on the formation of polycrystlline MgB2 and superconductivites were investigated. The results shows that nano-Al2O3 reacts with Mg to form nan-MgO and Al, which changes the formation sequence of MgB2, during the process of reaction bwteen Mg and B powder. The precipitated MgO particles restrict the growth of MgB2 grains and therefore increase the pinning effect of grain boundary. The MgO particles can also act as pinning centers to improve critical current density of MgB2, however, the pinning effect of MgO particles and superconductivity of MgB2 superconductor are destroyed following the increasement, local agglomeration and growth of MgO particles. Addtionally, Al atoms are partially substituted in the lattice of MgB2 at Mg sites to change the electronic structure of MgB2 and accordingly depress the critical transition temperature.
The precipitation of MgO as scondary phase is inevitable during in-situ sintering process for preparation of polycrystalline MgB2 superconductor. The formation, evolution and effect on superconductivity of MgO phase were explored. It was found that the precipitation of nano-scale MgO particles can be controlled by adjusting the holding time during the solid-solid reaction stage. These nano MgO particles remain till the solid-liquid stage and act as effective pinning centers on the surfaces and boundaries of large MgB2 grains to improve its superconductivity.
Eutectic Mg58.0Cu42.0 and Cu86.7B13.3 were adopted as precursors of Mg and B to prepare a ternary Mg15Cu75B10 alloy at a low temperature (1100℃) under ambient atmosphere by means of induce melting. Then, hyper-undercooling Mg15Cu75B10 alloys with different primary undercooling degrees were prepared by using cyclic overheating purification method. Based on the classical nucleation theory and transient nucleation theory, the thermodynamics and the kinetics for the competitive nucleation between MgB2 and MgB4 phase were calculated, and hence a critical condition for the formation of the MgB2 phase in hyper-undercooling Mg15Cu75B10 melt was proposed. The results indicate that MgB2 phase has smaller critical nucleation energy, higher nucleation rate and shorter incubation time comparing with MgB4 phase as the undercooling degree excesses to a critical value at a low undercooling range. Hence, the MgB2 phase nucleates primarily from the melt and grows up. However, in the high undercoolings (more than 223 K) range, both MgB2 and MgB4 phase have low nucleation rate and large incubation periods, so it is difficult for them to nucleate from undercooled melt.
利用高精度差热分析仪对Mg粉和B粉的混合试样进行烧结处理并同步监控其反应进程,在实验分析的基础上,结合经典烧结理论和多重扫描动力学分析方法,系统研究了原位烧结制备过程中多晶MgB2相的形成过程及成相机理,并确定了最佳烧结工艺。结果表明:Mg粉和B粉在Mg熔化之前便已开始反应,直到1023K时才反应完全并生成多晶MgB2相,因此在略高于1023K烧结得到的样品具有高的超导转变温度。在Mg粉熔化之前,反应属于固-固反应阶段,相互接触的Mg和B颗粒通过原子之间的相互扩散在接触处形成固溶活化区,MgB2相便在该活化区内形成。多重扫描动力学分析结果显示:固-固反应所对应的最概然机理函数为Avrami-Erofeev,表明MgB2相在该阶段受随机形核和随后的瞬时生长方式所控制,生成的MgB2颗粒以纳米尺度存在且很难长大;Mg粉熔化以后,反应进入固-液反应阶段,熔融Mg的渗透和包覆作用促进了Mg和残余B之间的反应,与此同时,MgB2晶粒在该阶段按Ostwald熟化机制迅速长成规则的六边形。
在上述工作基础上,同样采用热分析的手段,进一步研究了原位烧结过程中纳米Al2O3粉末掺杂对多晶MgB2相形成过程的影响,并确定了该影响与超导电性之间的内在联系。结果表明:在Mg粉和B粉原位烧结反应过程中,纳米Al2O3将与Mg发生反应而生成纳米MgO和单质Al,由此改变了MgB2相的形成进程。纳米MgO颗粒的析出限制了MgB2晶粒的长大,从而有效增加了MgB2多晶体的晶界磁通钉扎作用。同时,纳米MgO颗粒自身也可作为有效的磁通钉扎中心,提高MgB2多晶体的临界电流密度。但随着纳米Al2O3粉末掺杂量的增多,析出的纳米MgO颗粒将发生局部团聚和长大而失去磁通钉扎作用,从而破坏了MgB2多晶体的整体超导电性。Al原子部分取代MgB2晶体结构中Mg原子的位置,从而破坏了它的电子结构而显著降低MgB2多晶体的临界转变温度。
在原位烧结制备多晶MgB2超导体过程中,MgO作为第二相的析出不可避免。对自生MgO相的形成、演化规律及对超导电性能影响的研究发现:通过调整固-固反应阶段的保温时间可实现自生纳米MgO颗粒的控制析出,并将其保留到固-液反应阶段的大角度MgB2晶界及表面处形成有效的磁通钉扎中心,从而得到高超导电性能的多晶MgB2超导体。
选取共晶Mg58.0Cu42.0和Cu86.7B13.3作为Mg和B的先驱相,采用高频感应熔炼的方法在常压及较低温度(1100℃)条件下熔配Mg15Cu75B10三元合金。随后,利用循环过热的净化方法得到不同初始过冷度条件下的深过冷Mg15Cu75B10合金,并借助经典形核理论和瞬态形核理论,讨论了Mg15Cu75B10合金组织中MgB2和MgB4相之间竞争形核的热力学和动力学条件,给出了过冷Mg15Cu75B10合金组织中形成MgB2相的临界条件。结果表明:在小过冷度范围内,当过冷度大于某一临界过冷度时,MgB2相比MgB4相具有更低的形核功、更大的形核率和更短的形核孕育时间,因而MgB2相从与MgB4相的竞争形核中胜出,优先从合金熔体中形核并生长。然而,在大的初始过冷度(大于223K)下,MgB2相和MgB4相均具有较小的形核率和较长的形核孕育时间,很难从过冷熔体中形核。
The researches on the formation mechanism and controlling of magnesium diboride (MgB2) have become more and more important for guiding the preparation of high quality MgB2 superconductors by optimzing the prepration process. In the present paper, polycrystalline MgB2 superconductor and Mg-Cu-B ternay alloy were prepared by traditional solid-state sintering method and high frequency induction metling method respectively. Moreover, the evolution mechanisms of MgB2 phase during the in-situ sintering process and in the undercooed Mg-Cu-B alloy melt were systematically explored by means of metallographic analysis, diffrerential thermal analysis, powders sintering theory, thermodynamics and kinetics ananlysis. Additionally, the evolutions of ex-situ and in-situ formed nano-dopants during in-situ sintering process, as well as, the effects on the formation process and superconductivity of polycrystalline MgB2 were clarified. More details were given as follows.
A high-resolution Differerntial Thermal Analysis (DTA) apparatus was used to sinter the mixture of Mg and B powder and monitor the reaction process. Based on the classical sintering theory and multiple scan kinetic analysis method, the formation process and mechanism of polycrystalline MgB2 during in-situ sintering process were systemticlly investigated and the optimal sintering process was determined. It indicates that the reaction between Mg and B powders starts before Mg melting and not complete until the sintering temperature exceeds to 1023K, which is the best temperature to obtain MgB2 superconductor exhibiting the highest critical transition temperature (Tc). Solid-solid reaction stage (before Mg melting), the solution activated regions formed ahead at the contact areas between Mg and B particles by diffusion of atoms, then MgB2 phase precipitated from these regions. Furthermore, multiple scan kinetic analysis shows that the most probable mechanism function of this solid-solid reaction is Avrami–Erofeev equation, which represents that the formation of MgB2 phase is controlled by a mode of andom nucleation followed by instantaneous growth of nuclei. Therefore, the grains are in nano-scale at this stage. At the solid-liquid reaction stage (after Mg melting), the molten Mg promotes the reaction between Mg and residual B powders under the infilteration and enwrapping effects. Finally, the formed MgB2 grains grow up to regular hexagon morphology rapidly by Ostwald ripening mechanism.
Based on the above observations, the doping effect of nano-Al2O3 powders on the formation of polycrystlline MgB2 and superconductivites were investigated. The results shows that nano-Al2O3 reacts with Mg to form nan-MgO and Al, which changes the formation sequence of MgB2, during the process of reaction bwteen Mg and B powder. The precipitated MgO particles restrict the growth of MgB2 grains and therefore increase the pinning effect of grain boundary. The MgO particles can also act as pinning centers to improve critical current density of MgB2, however, the pinning effect of MgO particles and superconductivity of MgB2 superconductor are destroyed following the increasement, local agglomeration and growth of MgO particles. Addtionally, Al atoms are partially substituted in the lattice of MgB2 at Mg sites to change the electronic structure of MgB2 and accordingly depress the critical transition temperature.
The precipitation of MgO as scondary phase is inevitable during in-situ sintering process for preparation of polycrystalline MgB2 superconductor. The formation, evolution and effect on superconductivity of MgO phase were explored. It was found that the precipitation of nano-scale MgO particles can be controlled by adjusting the holding time during the solid-solid reaction stage. These nano MgO particles remain till the solid-liquid stage and act as effective pinning centers on the surfaces and boundaries of large MgB2 grains to improve its superconductivity.
Eutectic Mg58.0Cu42.0 and Cu86.7B13.3 were adopted as precursors of Mg and B to prepare a ternary Mg15Cu75B10 alloy at a low temperature (1100℃) under ambient atmosphere by means of induce melting. Then, hyper-undercooling Mg15Cu75B10 alloys with different primary undercooling degrees were prepared by using cyclic overheating purification method. Based on the classical nucleation theory and transient nucleation theory, the thermodynamics and the kinetics for the competitive nucleation between MgB2 and MgB4 phase were calculated, and hence a critical condition for the formation of the MgB2 phase in hyper-undercooling Mg15Cu75B10 melt was proposed. The results indicate that MgB2 phase has smaller critical nucleation energy, higher nucleation rate and shorter incubation time comparing with MgB4 phase as the undercooling degree excesses to a critical value at a low undercooling range. Hence, the MgB2 phase nucleates primarily from the melt and grows up. However, in the high undercoolings (more than 223 K) range, both MgB2 and MgB4 phase have low nucleation rate and large incubation periods, so it is difficult for them to nucleate from undercooled melt.
引文
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