Nonlinear surface impedance of YBCO thin films: Measurements, modeling, and effects in devices

High-T _c thin films continue to be of interest for passive device applications at microwave frequencies, but nonlinear effects may limit the performance. To understand these effects we have measured the nonlinear surface impedanceZ _s in a number of YBa₂Cu₃O_7–x thin films as a function of frequency from 1 to 18 GHz, rf surface magnetic fieldH _rf to 1500 Oe, and temperature from 4 K toT _c . The results at lowH _rf are shown to agree quantitatively with a modified coupled-grain model and at highH _rf with hysteresis-loss calculations using the Bean critical-state model applied to a thin strip. The loss mechanisms are extrinsic properties resulting from defects in the films. We also report preliminary measurements of the nonlinear impedance of Josephson junctions, and the results are related to the models of nonlinearZ _S . The implications of nonlinearZ _S for devices are discussed using the example of a five-pole bandpass filter.The authors thank Dr. Alfredo Anderson of Lincoln Laboratory for many YBCO films and technical discussions concerning film properties. We thank Dr. J. Halbritter for numerous discussions and ideas. We thank also Prof. S. Sridhar, B. Willemsen, N. Belk, and Dr. J. Herd for technical discussions. We are deeply indebted to R. P. Konieczka and D. J. Baker for resonator assembly and technical support. Drs. R. W. Ralston, T. C. L. G. Sollner, and P. Carr are thanked for technical guidance and support.     

In-situ deposition of superconducting YBCO thin films by magnetron sputtering from powder target

We have used stoichiometric Y₁Ba₂Cu₃O_7–x powder as magnetron sputtering target to deposit high-quality high-T _c superconducting thin films on MgO, SrTiO₃, and ZrO₂ substrates. The zero-resistance temperatures are 86–88 K, and the 77 K zero-field critical current density is 8 × 10⁵ A/cm². The films are highlyc-axis oriented. Films deposited on 10 × 10 × 1 mm³ ZrO₂ substrates have surface resistances below 25 m at 77 K and 94 GHz. Using powder targets instead of bulk targets has the following advantages: simple and low-cost target preparation, simple target replacement, and versatility for large-area deposition.     

Dielectric resonator as a possible standard for characterization of high temperature superconducting films for microwave applications

The performance of several designs of dielectric resonators used for microwave characterization of FITS films has been analyzed from the point of view of accuracy, sensitivity, and range. Designs discussed include Hakki-Coieman shielded types as well as open-ended resonators with sapphire, rutile and (ZrSn)TiO₃ dielectrics. The best dielectric resonators have proved to have an uncertainty in surface resistance measurements only twice the uncertainty in theQ-factor. high sensitivity, and ability to measure a wide range of surface resistances. Hence the dielectric resonator technique can be considered as a standard for measurements of surface resistance of HTS films for wireless and PCS communication systems applications provided that adequate measurement procedures of theQ ₀-factor are followed.     

Studies on microbridges of superconducting YBCO thin film

Critical current densities of the superconducting Y-Ba-Cu-O (YBCO) films have been observed to decrease with the increase of power of microwave radiation. Presence of Josephson type of junctions in the microbridges has been established from the microwave irradiation and magnetic field studies. BCS energy gap parameter (2/kT _c ) has been calculated from thedI/dV characteristics and found to be 3.7 at 13 K.     

Process parameters optimization for laser deposition of highT c superconducting thin films on Si and other substrate materials

Thin films of Y-Ba-Cu-O superconductor have been deposited on different substrates by pulsed excimer laser ablation from a superconducting pellet. The dependence of various process parameters such as substrate temperature, laser energy density, oxygen partial pressure, applied bias field and cooling rates on the quality of the films has been studied.     

Vaporization of a mixed precursor in chemical vapor deposition of YBCO films

Single-phase YBa₂Cu₃O_7– thin films withT _c values around 90 K are readily obtained by using a single-source chemical vapor deposition technique with normal precursor mass transport. The quality of the films is controlled by adjusting the carrier gas flow rate and the precursor feed rate.     

Zn^2＋掺杂的YBCO薄膜的超导性能研究

采用溶胶-凝胶法在LaAlO3（LAO）单晶衬底上制备了Zn^2＋掺杂的YBCO薄膜,用X射线衍射（XRD）、高分辨透射电子显微镜（HRTEM）以及标准四引线法分别研究了Zn^2＋掺杂的YBCO薄膜的微观结构、生长取向以及超导性能。结果表明,随Zn2＋掺杂浓度的增大薄膜的临界电流密度提高,而临界转变温度下降、临界转变温区变宽;但当掺杂量〉0.5%（摩尔分数）时,会影响YBCO的c轴取向生长,导致超导性能变差。通过优化掺杂比例后得到Zn2＋掺杂0.5%（摩尔分数）的YBCO薄膜具有最好的综合超导性能,其TC为91.3K,ΔT为1.1K,Jc约为1.54MA/cm^2（77K,0T）。     

YBa2Cu3O x superconducting thin films prepared by low temperature vacuum codeposition

Y-Ba-Cu-O films were prepared by low temperature codeposition of three components. The Y and Cu contents were evaporated from metallic sources, while Ba was vacuum-evaporated from Ba, BaO and BaF₂ sources in separate codeposition experiments. The lowest temperature at which superconducting YBa₂Cu₃O _x thin films (about 0.5μm thick) preparedin situ was near 500°C. This process enables preparation of superconducting films on various substrates (SrTiO₃, MgO, Al₂O₃, Si) without a buffer layer. Zero-resistance critical temperature was as high as 88 K and the critical current density was 10⁴ A/cm² at 4.2 K. The morphology of the films was granular with disordered grain orientation, the average grain size being typically 0.5μm.     

STO/YBCO薄膜的制备及介电性能研究

采用脉冲激光沉积技术,在(100)LaAlO3单晶基片上生长SrTiO3/Y1Ba2Cu3O7-x(STO/YBCO)多层薄膜。XRD分析表明:YBCO薄膜和STO薄膜均为C轴取向,STO(002)/YBCO(006)衍射峰摇摆曲线半高宽为0.73°。AFM分析表明,STO/YBCO多层薄膜表面平整、均匀,在77K,100kHz的测试条件下,STO薄膜介电损耗tgδ<10-2,在53.6kV/cm电场作用下,介电常数的相对变化为38%。     

Nano-assembled Pd catalysts on MgO thin films

Cluster-assembled materials open fascinating new routes for tuning physical and chemical properties by changing cluster size and often behave completely differently than their bulk analogues. By depositing gas phase Pd clusters on MgO thin films, model catalysts are fabricated which exhibit remarkable catalytic activity. In contrast to the high selectivity of Pd (111) surfaces for the cyclotrimerization of acetylene to benzene, small supported Pd_n clusters reveal a strongly size-dependent selectivity and catalyse the formation of benzene as well as of other hydrocarbons. The understanding at the atomistic level of the observed processes has been obtained by means of first-principles quantum–mechanical simulations. The theoretical studies have shown the importance of the surface defects of the oxide substrate in stabilizing the supported clusters but also in promoting their catalytic activity. For instance, Pd atoms bound at the regular sites of the MgO (100) terraces do not promote the acetylene to benzene conversion while they become active catalysts when bound at oxygen vacancies (F centers).     

Structural and superconducting properties of Nb-Ti alloy thin films

The structural instability in Ti rich compositions of Nb-Ti alloy system, which leads to various anomalies in the normal and superconducting state properties, has been extensively studied in bulk samples in the past. In this paper we report the formation of thin films of several compositions in the Ti rich region of this alloy system by RF magnetron sputtering and investigation of their electrical properties. Compositional analysis of two representative films was carried out by the RBS technique and the compositions agreed to within 2% of the targeted values. The anomalous variations in the electrical properties characteristic of the bulk, which can be ascribed to the structural instability related to the formation of athermal ω-phase, are also observed in thin films.     

PZT薄膜厚度对BMT/PZT复合薄膜结构及介电性能的影响

采用液相旋涂法制备了Ba(Mg1/3Ta2/3)O3(BMT)/Pb(Zr0.52Ti0.48)O3(PZT)复合薄膜,研究了PZT薄膜厚度对BMT/PZT复合薄膜结构及介电性能的影响。随着PZT薄膜厚度的增加,BMT/PZT复合薄膜的介电常数呈线性增加。当PZT薄膜的厚度较小时,会明显地增加BMT/PZT复合薄膜的介电损耗;当继续增加PZT薄膜的厚度,介电损耗反而下降直到与BMT薄膜的介电损耗值接近。这是由于PZT的介电常数与介电损耗均明显高于BMT薄膜所致,而异质界面的存在抑制了PZT薄膜中畴壁的运动,使其对复合薄膜介电损耗的影响减弱。研究结果表明,PZT薄膜的引入可以提升BMT薄膜的介电常数而对介电损耗的影响不大。     

Straight-sided, buckling-driven delamination of thin films at high stress levels

The fracture mechanics of a straight-sided, thin film delamination at stress levels, which are high compared to the stress required to initiate the delamination is investigated. Buckling at a bifurcation point of the delaminated region, resulting from incompletely relieved stresses in this region, is analysed by a semi- analytical approach for delaminations of infinite extent. The results are compared to numerical predictions based on finite element calculations for finite sized delaminations. The finite element calculations are carried out in the post-buckling regime showing that parts of the crack front will close as a result of bifurcation buckling, while other parts will experience enhanced energy release rate and mode I stress intensity factor. The mode III stress intensity factor is shown to be negligible at the stress levels analysed.     

Power handling capabilities of superconducting YBCO thin films: Thermally induced nonlinearity effects

We have measured the microwave power dependence of the surface impedance Z_s of YBa₂Cu₃O_x thin films up to very high microwave power levels. Films with different crystal qualities, including one with a bicrystal Josephson junction, were investigated. The experiments included both frequency-domain and pulsed time-domain measurements using a 14 GHz TE₀₁₁ dielectric cavity. Our results demonstrate that the dissipation of heat, generated by rf currents in the superconducting film, contributes to the observed nonlinearities in the surface resistance. The relative extent of this contribution is determined primarily by the film quality. A simple Fabry-Perot resonator model, combined with a cavity heat transfer model, was used to analyze the effects of such nonlinearities on the electromagnetic response of the dielectric cavity to a pulsed input signal.     

退火温度对Ge-SiO2薄膜结构的影响

用射频磁控溅射制备了Ge-SiO2薄膜，在N2的保护下对薄膜进行了不同温度的退火处理。使用傅里叶变换红外光谱分析（FTIR）技术，X射线衍射谱（XRD），X射线光电子能谱（XPS）分析样品的微结构，研究样品在退火中发生的结构，组分的变化，结果表明退火温度对薄膜的结构，尤其是薄膜中的GeO含量和GeO晶粒的大小的影响是显著的，并对其做了详尽的分析讨论。     

氧化锌薄膜的拉曼光谱研究

利用拉曼光谱结合X射线衍射分析对未掺杂和掺杂的ZnO薄膜，陶瓷薄膜进行了研究，ZnO薄膜及ZnO陶瓷薄膜均由sol-gel法制备，掺杂组份有Bi2O3,Sb2O3,MnO和Cr2O3等。结果表明，未掺杂的薄膜的ZnO主晶相均表现出显著的定向生长特征，其拉曼光谱特征谱峰为437cm^-1，谱峰强度随薄膜退火温度的提高略有增强，掺杂后ZnO的拉曼谱峰发生了红移，掺Bi2O3后ZnO的拉曼谱峰由347cm^-1移质移至434cm^-1，掺Sb2O3后ZnO的拉曼谱峰移至435cm^-1，而掺杂Bi2O3,Sb2O3,MnO和Cr2O3等组份的ZnO陶瓷薄膜的ZnO拉曼谱峰则移至434cm^-1，说明掺杂元素进入了ZnO晶格，引起了晶格的变化，ZnO薄膜性能不仅受次晶相组成的影响，而且受因掺杂元素进入而引起的ZnO晶格畸变的影响。     

衬底温度对沉积的NiTi薄膜晶化行为的影响

将采用直流磁控溅射方法制备的NiTi薄膜沉积在热的衬底上，应用X射线散射和小角X射线散射技术研究了NiTi合金薄膜中生成的晶化粒子的界面特征和晶粒大小．结果表明：衬底加热可降低薄膜的晶化温度，衬底温度在350℃以上，溅射的NiTi薄膜已部分晶化；衬底温度在350、370℃和420℃溅射的NiTi薄膜，对应的晶化粒子的半径分别是2．40、2．59、2．81nm；薄膜中的晶化粒子以形核长大的方式进行，结晶粒子与基底之间有清晰的界面。     

AFM study of the surface morphology of L-CVD SnO2 thin films

In this paper we present the results of Atomic Force Microscopy (AFM) characterisation of the surface morphology of the L-CVD SnO₂ thin films prepared by L-CVD technology and studied after exposure to air, dry air oxidation, and ion beam profiling. The L-CVD SnO₂ thin films after air exposure have a very smooth surface morphology with an average surface roughness (RMS) smaller than 0.5 nm, and average and maximal grain heights of about 1 and 2 nm, respectively. After dry air oxidation the L-CVD SnO₂ thin films exhibit an average surface roughness (RMS), as well as the average and maximal grain height, increased by one order of magnitude. Finally, after the ion beam profiling the L-CVD SnO₂ thin films exhibit an evidently disordered structure with a lot of craters. These experiments showed that the L-CVD SnO₂ thin films exhibit a very high quality surface morphology, what can be useful for solar cells and gas sensors application.     

先驱硼薄膜的质量对MgB2超导薄膜的影响

以高纯乙硼烷(B2H6)为B源,采用化学气相沉积(CVD)方法在多晶Al2O3衬底上沉积B薄膜,然后在Mg蒸气中异位退火来制备MgB2超导薄膜.通过X射线衍射和MgB2超导薄膜的电阻-温度曲线,研究了先驱硼薄膜的质量对MgB2超导薄膜的影响.     

Research on resistance characteristics of YBCO tape under short-time DC large current impact

Research of the resistance characteristics of YBCO tape under short-time DC large current impact is the foundation of the developing DC superconducting fault current limiter (SFCL) for voltage source converter-based high voltage direct current system (VSC-HVDC), which is one of the valid approaches to solve the problems of renewable energy integration. SFCL can limit DC short-circuit and enhance the interrupting capabilities of DC circuit breakers. In this paper, under short-time DC large current impacts, the resistance features of naked tape of YBCO tape are studied to find the resistance – temperature change rule and the maximum impact current. The influence of insulation for the resistance – temperature characteristics of YBCO tape is studied by comparison tests with naked tape and insulating tape in 77 K. The influence of operating temperature on the tape is also studied under subcooled liquid nitrogen condition. For the current impact security of YBCO tape, the critical current degradation and top temperature are analyzed and worked as judgment standards. The testing results is helpful for in developing SFCL in VSC-HVDC.