Tl-2223超导薄膜的快速烧结新工艺研究

New Rapid Sintering Process of Tl-2223 Superconducting Films

采用改进的快速升温烧结工艺在(001)取向的铝酸镧(LAO)衬底上制备Tl-2223超导薄膜,探索了在O_(2)环境下双温烧结过程中高温段烧结温度对超导薄膜晶体结构及超导性能的影响。X射线衍射(XRD)结果表明,烧结温度对Tl系超导薄膜的晶粒取向至关重要,采用合适的烧结温度可以制备出纯c轴取向生长的Tl-2223薄膜,且结晶质量良好;扫描电镜(SEM)图像显示超导薄膜表面平整,晶体结构致密,明显呈现出层状生长结构。907℃恒温2 min所制备的Tl-2223薄膜具有良好的超导性能,其临界转变温度(T_(c))可达115 K,临界电流密度(J_(c))约为1.6 mA·cm^(-2)(77 K,0 T)。该工艺简化了Tl-2223超导薄膜的制备过程,有效提高了实验效率。

Tl-based high temperature superconducting materials have high critical transition temperature,high critical current density,and strong moisture resistance.It has a great potential application value in both of the strong and weak electric fields.In particular,Tl_2Ba_2Ca_2Cu_3O_(10) (Tl-2223),which belongs to the non-ideal typeⅡsuperconductors,has the highest transition temperature (128 K) in Tl-based superconducting family.It is a superconducting material suitable for practical application.The growth of high-quality Tl-2223 superconducting film is the basis for its application.At present,it isn't easy to prepare pure c-axis-oriented Tl-2223 films on the single crystal substrates.Because,in the traditional sintering method,the heating rate is too slow (2~10℃·min^(-1)),so the precursor film passes through the phase-formed temperature of other superconducting phases with a low critical transition temperature (such as Tl-2212 and Tl-1212) for a long time.When the temperature reaches the formation temperature of the Tl-2223 phase,other superconducting phases generated at low temperatures cannot be completely transformed or eliminated.Only the mixed-phase films can be obtained.Moreover,a low heating rate will lead to a long time of the constant temperature of the film,resulting in poor surface morphology.To solve these problems,the improved rapid heating-up sintering process (referred to as the double-temperature sintering process) was used to grow Tl-2223 films on (001) oriented lanthanum aluminate (LAO) substrates in this paper.The effect of the high-temperature section in the sintering process on the phase formation of superconducting films was explored.In this process,the crystallization of the precursor film included two constant temperature processes at different temperatures.First,the temperature was raised from room temperature to a certain temperature of 885~907℃at 35℃·s^(-1),and kept for 2 min.Then the temperature was lowered to 865℃at20℃·s^(-1) for 10 min and the superconducting sample was obtained after natural cooling.Prior to the double-temperature sintering process,the preparation of Tl-2223 superconducting films by the single-temperature sintering process (only one constant temperature process) in oxygen environment was studied first.According to the test results,the films,prepared by the process in which the molar ratio of Tl to Ba in the pellets to provide thallium oxide vapor in the annealing step was 1.2∶2 and the sintering condition was 905℃for 10min,had Tl-1223,Tl-2223 and Tl-2212 phases simultaneously,and the diffraction peak intensity of each superconducting phase was low.The transmission light was used to observe the sample under the metallographic microscope,and part of the transmission light could be seen,indicating that the thallium element volatilizes seriously to form many holes in the film when sintered under this condition.With the increase of Tl content in the pellets,the diffraction peak of Tl-2212 gradually increased,while the diffraction peak of Tl-2223 gradually decreased.Therefore,it was not easy to control the sintering condition of the pure phase Tl-2223 films by the singletemperature sintering process.On this basis,Tl-2223 films was prepared by the double-temperature sintering method.The superconducting sample annealed at 885℃for 2 min was mixed-phase film.It meant that the Tl-2212 phase and the Tl-2223 phase formed simultaneously.When the sintering temperature was raised to 900℃for 2 min,the primary phase was Tl-2223 and only a small amount of Tl-2212 phase,and the purity of Tl-2223 phase was greatly improved.Continue to raise the temperature to 905℃for 2 min,the film contained only a very weak diffraction peak of Tl-2212 phase in addition to the main phase of Tl-2223.When the sintering condition was 907℃for 2 min,there were only (001) diffraction peaks of Tl-2223 superconducting phase and LAO substrate,which indicated that the other superconducting phases had been completely transformed into Tl-2223 phase.Scanning electron microscope(SEM) image showed that the surface of the pure c-axis-oriented Tl-2223 films was the flat,dense crystal structure,and layered growth structure.The critical transition temperature (T_c) of the sample was measured by the mutual inductance method,and its value was about 115 K.The microbridge with a length of 100μm and a width of 40μm was fabricated on the films by etching method.The critical current of the sample was about 380 mA measured by standard four-electrode technique at liquid nitrogen temperature and zero magnetic fields.After calculation,the critical current density (J_c) value of the sample was about 1.6 mA·cm^(-2).The above experimental results showed that the sintering temperatures were crucial for the grain orientation of Tl-based superconducting films.The superconducting films prepared by suitable sintering temperatures had good phase purity and crystallization quality.This method had the advantages of the fast speed of rising and falling temperature,short sintering time,reduced costs and high experimental repetition rate.