铜活化二硼化镁低温烧结机制及超导电性
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摘要
制备具有高临界电流密度的二硼化镁超导体是当前超导研究的热点,近几年发展起来的低温烧结技术为显著改善临界电流密度提供了可能。由于低温烧结过程中原子在固相中扩散缓慢,MgB_2的合成一般都需要很长时间才能完成,因此寻找一种低廉有效的方法来提高烧结效率,在低温条件下快速合成具有更高临界电流密度的MgB_2超导材料成为目前亟待解决的问题。在此背景下,本文首先阐明了低温条件下MgB_2的活化烧结机理和掺杂机制;以此为指导,把有效碳掺杂和球磨自氧化技术成功地应用到Cu活化烧结MgB_2的低温制备中;利用这两种新技术在低温下快速合成具有优异临界电流密度的MgB_2超导体,基本可满足应用要求。上述研究包含的主要内容及获得的结论有:
综合采用原位X-射线衍射和扫描示差量热分析方法对Mg-B体系从低温到高温烧结全过程进行了探究,其后结合等温动力学分析重点阐明了低温成相动力学机理。结果表明,Mg-B体系低温固相反应是由可变的动力学机制控制,反应刚开始阶段活化的Mg和B原子较少,反应速率比较慢,该阶段主要为相界面控制的反应过程;随着反应的进行,生成的MgB_2层越来越厚,Mg的扩散变得越来越困难,此时一维扩散成为控制性环节。上述动力学机理导致反应激活能随着反应进行先减小而后增大。
系统研究了不同金属(Cu、Sn和Ag)添加对MgB_2低温烧结效率的影响,据此澄清了金属活化低温烧结MgB_2的机理,并提出了金属活化剂选取的判据。结果表明:在Mg-Cu-B体系中,Cu首先与Mg在485℃左右形成Mg-Cu共晶液相,它可为Mg向B的扩散提供快速通道,降低MgB_2相的生成温度,促进MgB_2的生成。添加Cu的MgB_2烧结过程符合活化烧结机理。Ag和Sn的添加均能形成低温液相,明显提高MgB_2低温烧结效率,但对MgB_2低温烧结效率的提高比添加Cu弱。结合活化烧结机理分析确定了微量Cu作为最佳活化剂。
结合微观组织形貌观察和超导性能测试全面分析了在低温和高温烧结条件下活化剂Cu对MgB_2微观组织和超导性能的影响。活化剂Cu在MgB_2烧结过程中能够有效减少MgO杂质的生成,提高低磁场下的临界电流密度。在较低的烧结温度下,局部Mg-Cu液相的生成能够溶解和包覆部分Mg颗粒,隔绝了它们与气态氧原子或者固态含氧物质的接触,因此可以减少它们的氧化。烧结温度在Mg的熔点以上时,Cu的加入可明显降低Mg熔体的蒸气压,减少Mg的挥发和气化Mg原子的数量,最终减少了因气化Mg原子与保护气氛中残余氧气反应生成的MgO杂质的数量。不同添加量的Cu活化低温烧结制备的MgB_2块体的显微组织形态差别明显。适量Cu添加可以生成层状MgB_2晶体组织,它取决于活化剂Cu添加后形成的Mg-Cu液相烧结环境。在该液相烧结环境中,MgB_2晶体能够以二维形核长大的机制最终形成层状组织。与传统高温烧结形成的典型MgB_2晶体组织相比,层状MgB_2晶体组织具有更优异的晶间连接性。
为了进一步提高Cu活化低温烧结制备的MgB_2的载流能力,本文还系统分析了低温引入碳掺杂的机制,提出了碳掺杂在Cu活化低温烧结过程中有效引入的判据。研究表明,碳基化合物低温引入碳掺杂的关键是能够在MgB_2相生成之前或者同时释放出活性足够高的C。释放C的活性决定了低温引入碳掺杂的有效性。据此分析了纳米SiC和葡萄糖的低温引入碳掺杂机制的可行性:在烧结过程中,纳米SiC在低温下通过叠加反应释放出活性足够高的C,形成有效碳掺杂;葡萄糖低温分解释放出的活性C相对较少,低温时很难形成有效碳掺杂。在Cu活化低温烧结中能够引入有效碳掺杂的碳基化合物应该不与Cu反应,不破坏Cu的活化烧结作用。反过来,Cu的活化烧结也能够促进或者是至少不阻碍该碳基化合物低温有效引入碳掺杂。根据此条件,选取了纳米SiC为合适掺杂剂,在低温活化烧结中快速合成了碳掺杂MgB_2块体。无论是在低磁场中还是在高磁场中,其临界电流密度值都有明显的提高。
除了引入碳掺杂以外,还尝试使用球磨处理和Cu活化烧结相结合的方法来进一步提高低温烧结效率和相应的临界电流密度。在这个过程中,发现短时间球磨后粉末的微量氧化不仅不恶化临界电流密度,反而会提高高场下的临界电流密度大小。在系统分析该现象的基础上总结出球磨自氧化引入MgO掺杂的机制,并把这一机制应用到Cu活化低温烧结MgB_2中,最终发现其在中低磁场下的载流能力有了更优异的表现。究其原因,一方面球磨自氧化机制可以促进Cu活化低温烧结,提高烧结试样的致密度,使得MgB_2晶粒排列紧密,有些甚至呈层状排列,这些因素都显著优化了MgB_2的晶间连接性;另一方面球磨自氧化机制在细化MgB_2晶粒的同时还可引入大量均匀分布的纳米MgO颗粒,显著增加了有效磁通钉扎中心。球磨自氧化处理与Cu活化低温烧结技术相结合,可以从提高晶间连接性和增加磁通钉扎中心两方面共同改善载流能力,最终大幅度提高了临界电流密度值,基本达到实际应用水平。
The synthesis of MgB_2 with high critical current density is the highlight of superconducting field. Low-temperature sintering developed in recent years is a potentially effective method for obtaining improved J_c. However, the reaction between magnesium and boron at low temperature takes a very long time to form the complete MgB_2 phase as the result of the low diffusion rate of atoms in the solid-state. Hence, to search a convenient and effective way in improving the sintering efficiency together with increasing J_c is very urgent. Based on these backgrounds, the activated sintering mechanism as well as doping mechanism in MgB_2 prepared at low temperature was explored in present work. Accordingly, the techniques that both autoxidation treatment of short-time milled original powders and effective C doping were introduced into the Cu-activated sintering process of MgB_2 at low temperature. As a result, excellent J_c was obtained in the synthesized samples, which basically meet the practical need of MgB_2 superconductors.
Combined with in-situ X-ray diffraction and thermal analysis, the sintering process of Mg-B system from low temperature to high temperature was investigated. Then the kinetic mechanism of MgB_2 phase formation at low temperature was clarified according to the isothermal kinetics calculation. The result indicated that the reaction between Mg and B during the low-temperature sintering is controlled by varied mechanisms. At initial stage, the activated Mg and B atoms are limited and the reaction rate is mainly determined by the phase boundary reaction mechanism. As the reaction prolonging and the synthesized MgB_2 layer increasing, the diffusion-limited mechanism gradually becomes dominant. The corresponding activation energy is also decreased firstly and then increased again.
The influence of different metals (Cu, Sn and Ag) on the sintering efficiency of MgB_2 at low temperature was systemically studied. Accordingly, the metal activated sintering mechanism of MgB_2 at low temperature was clarified and also the criteria for selecting appropriate activator were proposed. It was found that during the sintering process of Mg-Cu-B system, Cu firstly reacted with Mg forming Mg-Cu liquid, which could provide high transport for the diffusion of Mg atoms into B and thus accelerate the MgB_2 phase formation. The effect of Cu addition on the sintering process was proved to follow the activated sintering mechanism. Besides, Ag and Sn also formed local liquid at low temperature and thus improved the sintering efficiency of MgB_2 obviously. However, the enhancement in the sintering efficiency resulting from Ag and Sn addition is not as significant as Cu addition. This phenomenon can be explained well on the basis of activated sintering mechanism. The minor Cu addition is finally determined to be the most suitable activator according to above results.
The effect of Cu activator on the microstructure and superconductive properties of MgB_2 sintered at both low temperature and high temperature were investigated in detail. Results showed that the Cu activator could reduce the amount of MgO impurity in the prepared MgB_2 samples dramatically. During the low-temperature sintering process, the formation of local Mg-Cu liquid could wrap the Mg particles and thus protect them from the oxidation caused by the oxide impurity in the starting powders and the gaseous O; During the high-temperature sintering process, the Cu addition could decrease the vapor pressure of Mg liquid and thus reduce the volatilization of Mg liquid, which could depress the reaction between gaseous Mg and O_2 impurity in the protective Ar gas forming MgO impurity. It is also found that the lamellar MgB_2 grains could be synthesized by Cu activated sintering at low temperature. It is proposed that the Mg-Cu liquid environment is the critical condition, and the lamellar MgB_2 grains was formed at the liquid-solid interface between Mg-Cu liquid and B following the two dimensional nucleation and growth mechanism. According to the analysis on the temperature dependence of resistivity, the lamellar MgB_2 grains possess much better grain connectivity than the typical morphology of MgB_2 grains synthesized by the traditional solid-state sintering.
In order to further optimize J_c in the MgB_2 synthesized by Cu activated sintering at low temperature, the C doping mechanism in the carbon-based chemicals added MgB_2 at low temperature was discussed and then the criteria for judging validity of C doping during the Cu activated sintering process of MgB_2 were proposed. The results indicated that following factors should be considered when discussing the validity of C doping at low temperature: (ⅰ) carbon-based chemicals ought to release C before or simultaneously with the formation of the MgB_2 phase. (ⅱ) The released C should be active enough to guarantee that it could enter the MgB_2 crystal lattice and replace B atoms. Accordingly, the validity of C doping resulting from nano SiC and glucose (C_6H_(12)O_6) at low temperature were verified: Nano SiC could release related high active C via reaction between Mg and SiC at low temperature and then introduce C doping effectively while C6H12O6 could not release sufficiently high active C through its decomposition and failed to bring about C doping. The carbon-based chemical which will introduce C doping effectively into MgB_2 sample prepared by Cu activated sintering at low temperature must meet two criteria as follows: (ⅰ) It should not react with Cu during the low temperature sintering process, and therefore not destroy its effect of activated sintering. (ⅱ) On the other hand, Cu should improve or at least not inhibit C doping caused by this carbon-based chemical addition. In view of above, nano SiC was selected as appropriate dopant. Consequently, the corresponding SiC doped MgB_2 samples activated sintered at low temperature exhibit excellent J_c.
Besides C doping, ball milling treatment of original powders was also applied to further increase J_c in MgB_2 samples prepared by Cu activated sintering at low temperature. It is observed that the slight oxidation of short-time milled original powders led to the increase of J_c at high field. Based on the investigation of this phenomenon, the autoxidation technique of milled original powders was developed and then introduced into the Cu activated sintering process of MgB_2 at low temperature. On one hand, autoxidation treatment of milled original powders could improve Cu activated sintering and result in higher density. Moreover, some lamellar MgB_2 grains were observed in the Cu-activated sintered sample after autoxidation treatment of milled original powders. Both of factors could improve the connectivity between MgB_2 grains; On the other hand, autoxidation treatment of milled original powders could generate lots of homogenously distributed MgO nanoimpurities as well as refine MgB_2 grains in the sintered sample, which both significantly increase the number of flux pinning centers. Hence, the J_c in the MgB_2 sample prepared by combining autoxidation treatment of milled original powders with Cu activated sintering at low temperature was dramatically improved.
综合采用原位X-射线衍射和扫描示差量热分析方法对Mg-B体系从低温到高温烧结全过程进行了探究,其后结合等温动力学分析重点阐明了低温成相动力学机理。结果表明,Mg-B体系低温固相反应是由可变的动力学机制控制,反应刚开始阶段活化的Mg和B原子较少,反应速率比较慢,该阶段主要为相界面控制的反应过程;随着反应的进行,生成的MgB_2层越来越厚,Mg的扩散变得越来越困难,此时一维扩散成为控制性环节。上述动力学机理导致反应激活能随着反应进行先减小而后增大。
系统研究了不同金属(Cu、Sn和Ag)添加对MgB_2低温烧结效率的影响,据此澄清了金属活化低温烧结MgB_2的机理,并提出了金属活化剂选取的判据。结果表明:在Mg-Cu-B体系中,Cu首先与Mg在485℃左右形成Mg-Cu共晶液相,它可为Mg向B的扩散提供快速通道,降低MgB_2相的生成温度,促进MgB_2的生成。添加Cu的MgB_2烧结过程符合活化烧结机理。Ag和Sn的添加均能形成低温液相,明显提高MgB_2低温烧结效率,但对MgB_2低温烧结效率的提高比添加Cu弱。结合活化烧结机理分析确定了微量Cu作为最佳活化剂。
结合微观组织形貌观察和超导性能测试全面分析了在低温和高温烧结条件下活化剂Cu对MgB_2微观组织和超导性能的影响。活化剂Cu在MgB_2烧结过程中能够有效减少MgO杂质的生成,提高低磁场下的临界电流密度。在较低的烧结温度下,局部Mg-Cu液相的生成能够溶解和包覆部分Mg颗粒,隔绝了它们与气态氧原子或者固态含氧物质的接触,因此可以减少它们的氧化。烧结温度在Mg的熔点以上时,Cu的加入可明显降低Mg熔体的蒸气压,减少Mg的挥发和气化Mg原子的数量,最终减少了因气化Mg原子与保护气氛中残余氧气反应生成的MgO杂质的数量。不同添加量的Cu活化低温烧结制备的MgB_2块体的显微组织形态差别明显。适量Cu添加可以生成层状MgB_2晶体组织,它取决于活化剂Cu添加后形成的Mg-Cu液相烧结环境。在该液相烧结环境中,MgB_2晶体能够以二维形核长大的机制最终形成层状组织。与传统高温烧结形成的典型MgB_2晶体组织相比,层状MgB_2晶体组织具有更优异的晶间连接性。
为了进一步提高Cu活化低温烧结制备的MgB_2的载流能力,本文还系统分析了低温引入碳掺杂的机制,提出了碳掺杂在Cu活化低温烧结过程中有效引入的判据。研究表明,碳基化合物低温引入碳掺杂的关键是能够在MgB_2相生成之前或者同时释放出活性足够高的C。释放C的活性决定了低温引入碳掺杂的有效性。据此分析了纳米SiC和葡萄糖的低温引入碳掺杂机制的可行性:在烧结过程中,纳米SiC在低温下通过叠加反应释放出活性足够高的C,形成有效碳掺杂;葡萄糖低温分解释放出的活性C相对较少,低温时很难形成有效碳掺杂。在Cu活化低温烧结中能够引入有效碳掺杂的碳基化合物应该不与Cu反应,不破坏Cu的活化烧结作用。反过来,Cu的活化烧结也能够促进或者是至少不阻碍该碳基化合物低温有效引入碳掺杂。根据此条件,选取了纳米SiC为合适掺杂剂,在低温活化烧结中快速合成了碳掺杂MgB_2块体。无论是在低磁场中还是在高磁场中,其临界电流密度值都有明显的提高。
除了引入碳掺杂以外,还尝试使用球磨处理和Cu活化烧结相结合的方法来进一步提高低温烧结效率和相应的临界电流密度。在这个过程中,发现短时间球磨后粉末的微量氧化不仅不恶化临界电流密度,反而会提高高场下的临界电流密度大小。在系统分析该现象的基础上总结出球磨自氧化引入MgO掺杂的机制,并把这一机制应用到Cu活化低温烧结MgB_2中,最终发现其在中低磁场下的载流能力有了更优异的表现。究其原因,一方面球磨自氧化机制可以促进Cu活化低温烧结,提高烧结试样的致密度,使得MgB_2晶粒排列紧密,有些甚至呈层状排列,这些因素都显著优化了MgB_2的晶间连接性;另一方面球磨自氧化机制在细化MgB_2晶粒的同时还可引入大量均匀分布的纳米MgO颗粒,显著增加了有效磁通钉扎中心。球磨自氧化处理与Cu活化低温烧结技术相结合,可以从提高晶间连接性和增加磁通钉扎中心两方面共同改善载流能力,最终大幅度提高了临界电流密度值,基本达到实际应用水平。
The synthesis of MgB_2 with high critical current density is the highlight of superconducting field. Low-temperature sintering developed in recent years is a potentially effective method for obtaining improved J_c. However, the reaction between magnesium and boron at low temperature takes a very long time to form the complete MgB_2 phase as the result of the low diffusion rate of atoms in the solid-state. Hence, to search a convenient and effective way in improving the sintering efficiency together with increasing J_c is very urgent. Based on these backgrounds, the activated sintering mechanism as well as doping mechanism in MgB_2 prepared at low temperature was explored in present work. Accordingly, the techniques that both autoxidation treatment of short-time milled original powders and effective C doping were introduced into the Cu-activated sintering process of MgB_2 at low temperature. As a result, excellent J_c was obtained in the synthesized samples, which basically meet the practical need of MgB_2 superconductors.
Combined with in-situ X-ray diffraction and thermal analysis, the sintering process of Mg-B system from low temperature to high temperature was investigated. Then the kinetic mechanism of MgB_2 phase formation at low temperature was clarified according to the isothermal kinetics calculation. The result indicated that the reaction between Mg and B during the low-temperature sintering is controlled by varied mechanisms. At initial stage, the activated Mg and B atoms are limited and the reaction rate is mainly determined by the phase boundary reaction mechanism. As the reaction prolonging and the synthesized MgB_2 layer increasing, the diffusion-limited mechanism gradually becomes dominant. The corresponding activation energy is also decreased firstly and then increased again.
The influence of different metals (Cu, Sn and Ag) on the sintering efficiency of MgB_2 at low temperature was systemically studied. Accordingly, the metal activated sintering mechanism of MgB_2 at low temperature was clarified and also the criteria for selecting appropriate activator were proposed. It was found that during the sintering process of Mg-Cu-B system, Cu firstly reacted with Mg forming Mg-Cu liquid, which could provide high transport for the diffusion of Mg atoms into B and thus accelerate the MgB_2 phase formation. The effect of Cu addition on the sintering process was proved to follow the activated sintering mechanism. Besides, Ag and Sn also formed local liquid at low temperature and thus improved the sintering efficiency of MgB_2 obviously. However, the enhancement in the sintering efficiency resulting from Ag and Sn addition is not as significant as Cu addition. This phenomenon can be explained well on the basis of activated sintering mechanism. The minor Cu addition is finally determined to be the most suitable activator according to above results.
The effect of Cu activator on the microstructure and superconductive properties of MgB_2 sintered at both low temperature and high temperature were investigated in detail. Results showed that the Cu activator could reduce the amount of MgO impurity in the prepared MgB_2 samples dramatically. During the low-temperature sintering process, the formation of local Mg-Cu liquid could wrap the Mg particles and thus protect them from the oxidation caused by the oxide impurity in the starting powders and the gaseous O; During the high-temperature sintering process, the Cu addition could decrease the vapor pressure of Mg liquid and thus reduce the volatilization of Mg liquid, which could depress the reaction between gaseous Mg and O_2 impurity in the protective Ar gas forming MgO impurity. It is also found that the lamellar MgB_2 grains could be synthesized by Cu activated sintering at low temperature. It is proposed that the Mg-Cu liquid environment is the critical condition, and the lamellar MgB_2 grains was formed at the liquid-solid interface between Mg-Cu liquid and B following the two dimensional nucleation and growth mechanism. According to the analysis on the temperature dependence of resistivity, the lamellar MgB_2 grains possess much better grain connectivity than the typical morphology of MgB_2 grains synthesized by the traditional solid-state sintering.
In order to further optimize J_c in the MgB_2 synthesized by Cu activated sintering at low temperature, the C doping mechanism in the carbon-based chemicals added MgB_2 at low temperature was discussed and then the criteria for judging validity of C doping during the Cu activated sintering process of MgB_2 were proposed. The results indicated that following factors should be considered when discussing the validity of C doping at low temperature: (ⅰ) carbon-based chemicals ought to release C before or simultaneously with the formation of the MgB_2 phase. (ⅱ) The released C should be active enough to guarantee that it could enter the MgB_2 crystal lattice and replace B atoms. Accordingly, the validity of C doping resulting from nano SiC and glucose (C_6H_(12)O_6) at low temperature were verified: Nano SiC could release related high active C via reaction between Mg and SiC at low temperature and then introduce C doping effectively while C6H12O6 could not release sufficiently high active C through its decomposition and failed to bring about C doping. The carbon-based chemical which will introduce C doping effectively into MgB_2 sample prepared by Cu activated sintering at low temperature must meet two criteria as follows: (ⅰ) It should not react with Cu during the low temperature sintering process, and therefore not destroy its effect of activated sintering. (ⅱ) On the other hand, Cu should improve or at least not inhibit C doping caused by this carbon-based chemical addition. In view of above, nano SiC was selected as appropriate dopant. Consequently, the corresponding SiC doped MgB_2 samples activated sintered at low temperature exhibit excellent J_c.
Besides C doping, ball milling treatment of original powders was also applied to further increase J_c in MgB_2 samples prepared by Cu activated sintering at low temperature. It is observed that the slight oxidation of short-time milled original powders led to the increase of J_c at high field. Based on the investigation of this phenomenon, the autoxidation technique of milled original powders was developed and then introduced into the Cu activated sintering process of MgB_2 at low temperature. On one hand, autoxidation treatment of milled original powders could improve Cu activated sintering and result in higher density. Moreover, some lamellar MgB_2 grains were observed in the Cu-activated sintered sample after autoxidation treatment of milled original powders. Both of factors could improve the connectivity between MgB_2 grains; On the other hand, autoxidation treatment of milled original powders could generate lots of homogenously distributed MgO nanoimpurities as well as refine MgB_2 grains in the sintered sample, which both significantly increase the number of flux pinning centers. Hence, the J_c in the MgB_2 sample prepared by combining autoxidation treatment of milled original powders with Cu activated sintering at low temperature was dramatically improved.
引文
[1] J. Nagamatsu, N. Nakagawa, T. Muranaka et al., Superconductivity at 39K in magnesium diboride, Nature, 2001, 410: 63~64.
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