Microwave Surface Resistance Research Articles

High-Temperature Measurement Technology for Microwave Surface Resistance of Metal Materials

The resonant cavity method is a commonly used method for high-temperature testing of the complex permittivity of dielectric materials. When a resonant cavity is used for high-temperature testing, the microwave surface resistance of the cavity metal material will deteriorate due to factors such as oxidation reaction and thermal fatigue, resulting in a decrease in testing accuracy and repeatability. Therefore, when designing a high-temperature resonant cavity, the temperature response characteristics of the microwave surface resistance of the cavity metal material should be obtained in advance. In this paper, a high-temperature measurement method of microwave surface resistance of metal materials based on a separate cylindrical resonator is proposed, a mathematical model of microwave surface resistance inversion based on the resonator quality factor is established, and a high-temperature measurement system of microwave surface resistance is integrated. The reliability of the proposed method and system is verified through simulation and experiment. The measurement frequency covers 7-18 GHz, and the maximum test temperature reaches 500°C. Systematic error of microwave surface resistance measurement at room temperature is less than 3%.

A new technique to achieve thick Tl2Ba2CaCu2O8 films for advanced applications

Tl2Ba2CaCu2O8 (Tl-2212) high-temperature superconductors (HTS) exhibit excellent superconductivity performance as their critical temperature (Tc) is higher than 100 K. However, the emergence of microcracks on the film surface restricts the thickness of the obtained Tl-2212 films, thus greatly limiting their scientific research and engineering application. Here, we fabricate crack-free Tl2Ba2CaCu2O8 (Tl-2212) films with a thickness of up to 5 μm on LaAlO3 (001) substrates synthesized by an improved strategy of a closed system. The impact of the thickness of the Tl-2212 films on their crystallographic orientations, Tc, critical current density (Jc) (0 T), critical current intensity (Ic) (0 T), and microwave surface resistance (Rs) was systemically explored. As the thickness increases, Tc values fluctuate slightly above 100 K while the variation of Jc follows the extended strong pinning theory. Ic of the 4 μm thick film stays around 80 A at 95 K, and the Rs result is less than 1 mΩ at 93 K, indicating their excellent performance at high temperatures. The interaction mechanism between the Jc and Rs of the Tl-2212 film is replenished in light of the quantitative exploration of Jc and Rs. This work on Tl-2212 films will promote their fundamental physics research and guide their potential applications in the electronics and electrical field. It can also serve as a reference for future research on other HTS materials.

Microstructure study of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-<i>δ</i></sub> thin film with synchrotron-based three-dimensional reciprocal space mapping

High-temperature superconducting films can be used for fabricating the cutting-edge high-temperature superconducting microwave devices because of their low microwave surface resistances. However, the microwave surface resistances of high-temperature superconducting materials are particularly sensitive to microstructure due to their special two-dimensional superconducting mechanisms and extremely short superconducting coherence lengths. To investigate the correlations between microstructure and microwave surface resistance of high-temperature superconducting materials, YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-<i>δ</i></sub> (YBCO) films with different thickness are grown on (00l)-oriented MgO single-crystal substrates by using the pulsed laser deposition (PLD) technique. Electrical measurements reveal that their superconducting transition temperatures and room temperature resistances do not show significant difference. However, their microwave surface resistances in superconducting state display a significant difference. The characterizations of the microstructures of YBCO films by synchrotron radiation three-dimensional reciprocal space mapping(3D-RSM) technique show that the number of the grains with CuO<sub>2</sub> face parallel to the surface (c crystals), and the consistency of grain orientation are the main causes for the difference in microwave surface resistance.

Microwave surface resistance in nanostructured high-Tc superconductor films

The impact of artificially created defects nanostructure, formed by implanted dielectric nanoparticles or irradiation defects, on microwave properties of high-Tc superconductor films is analyzed in the framework of phenomenological theory for microwave response of type-II superconductors. We have calculated the surface resistance for such a kind of nanostructured type-II superconductor film and investigated conditions for the emergence of nonlinear response caused by the entrance of microwave-induced vortices in the film's interior through its edges. The obtained results indicate that artificial defect nanostructure in the film's interior formed by point-like or columnar structural defects can significantly improve its microwave characteristics in both the Meissner and mixed states and also increase the threshold for the onset of nonlinear response.

Composition effect on transport and microwave properties of YBCO thin films prepared by metalorganic chemical vapor deposition

YBa2Cu3O7−δ (YBCO) thin films deposited on LaAlO3 single-crystal substrates have great potential for application in microwave field due to their low microwave surface resistance (Rs). In order to prepare high-performance films and facilitate adjustment of experimental parameters, we designed novel metalorganic chemical vapor deposition system. This system adopts single liquid-source pulse feed that provides facile control over film composition through the adjustment of Y, Ba, and Cu ratios in precursors. Here, we thoroughly study influence of film composition on performance. By conducting energy-dispersive spectroscopy measurements, Y, Ba, and Cu composition ranges of 21–24%, 28–31%, and 46–54%, respectively, are found to optimize YBCO film transport and microwave properties. Composition affects purity of film phase which in turns determines film performance. To confirm this observation, we adjust ratio through input–output relationship to obtain film with composition ratio within abovementioned optimal range. Obtained YBCO film has critical current density of 2.8 MA/cm2 at 77 K and 0 T and Rs of 0.285 mΩ at 77 K and 10 GHz. The full width at half maximum of YBCO (005) peak and (103) plane, measured via X-ray diffraction, are 0.235° and 0.762°, respectively.

Evaluation of the nonlinear surface resistance of REBCO coated conductors for their use in the FCC-hh beam screen

To assess the feasibility of using high-temperature superconductors for the beam screens of future circular colliders, we have undertaken a study of the power dependence of the microwave surface resistance in state-of-the-art REBCO coated conductors at about 8 GHz and 50 K. We have employed a dielectric resonator to produce radio-frequency (RF) electromagnetic fields on the surface of the coated conductors having amplitudes similar to those generated by proton bunches circulating in the vacuum chamber of the proposed future circular collider Hadron-Hadron (FCC-hh) at CERN We show that surface resistances in REBCO coated conductors without artificial pinning centers are more affected by a RF magnetic field than those containing nano-inclusions. Despite that, at 8 GHz, 50 K, and 9 T, most REBCO coated conductors studied outperform copper in terms of surface resistance, with the best sample having a 2.3 mΩ surface resistance while being subject to an RF field 2.5 times stronger than that in the FCC-hh. We also extrapolate the measured data to 16 T and 1 GHz, the actual FCC-hh dipole magnetic field, and the mid-beam frequency spectrum, demonstrating the possibility of lowering the surface resistance of the vacuum chamber by up to two orders of magnitude compared to copper. Further, we discuss the correlation between the time structure of the electromagnetic fields provided by vector network analyzers compared to the proton bunches’ time structure in the collider and present the effect of low alternating magnetic fields on vortex displacement and the possibility of demagnetization of superconducting samples.

Quality Factor Enhancement of 650 MHz Superconducting Radio-Frequency Cavity for CEPC

Medium-temperature (mid-T) furnace baking was conducted at 650 MHz superconducting radio-frequency (SRF) cavity for circular electron positron collider (CEPC), which enhanced the cavity unloaded quality factor (Q0) significantly. In the vertical test (2.0 K), Q0 of 650 MHz cavity reached 6.4 × 1010 at 30 MV/m, which is remarkably high at this unexplored frequency. Additionally, the cavity quenched at 31.2 MV/m finally. There was no anti-Q-slope behavior after mid-T furnace baking, which is characteristic of 1.3 GHz cavities. The microwave surface resistance (RS) was also studied, which indicated both very low Bardeen–Cooper–Schrieffer (BCS) and residual resistance. The recipe of cavity process in this paper is simplified and easy to duplicate, which may benefit the SRF community.

Lift-Off and Tilt Effect in Microwave Surface Resistance Measurement Using TE011 Mode Cylindrical Cavity

Measurement of surface resistance at microwave or higher frequency bands is needed for applications, such as accelerators, microwave systems, and integrated circuits. Cylindrical cavity resonators operating at TE₀₁₁ mode have been applied in surface resistance measurement for decades. It is common that samples under test are usually used as one of the cavity&#x2019;s endplates and contact the cylinder side wall directly, which cannot be regarded as noncontact measurement. In this article, through quantitative research on lift-off effect of samples under test, we explore the possibility of realizing noncontact surface resistance measurement by intentionally introducing an air gap (ranging from &#x007E;0 to 10 mm) between the samples and the cavity side wall. From the results of both electromagnetic simulations and experiments, it can be concluded that by taking care of the sample&#x2019;s tilt effect, surface resistance can be measured in a noncontact manner. Measurement results of polished metal plates with electrical conductivity ranging from &#x007E;2 to 58 MS/m (measured with standard eddy current method) show that the relative measurement error is expected to be less than 10&#x0025;. Compared with related publications, the focus of this work is put on the lift-off effect and a larger range of air gap is introduced. The obtained results indicate that the air-gap size may be an additional consideration in the design of surface resistance measurement system for special applications, such as temperature-dependent measurements in vacuum chamber, <i>in situ</i> diagnosis tools.

Effect of Build Orientation and Laser Power on Microwave Loss in Metal Additive Manufactured Components

The adoption of metal additive manufacturing into the production of passive microwave components is still in its relative infancy. However, it is of increasing interest due to the offer of geometrical design freedom and significant weight reduction. The electrical properties of additive manufactured components are still inferior to traditional manufacturing techniques owing to the poor surface finish, especially on overhanging surfaces, which are unavoidable in three-dimensional microwave components. In this paper we present experimental findings on the disparity in microwave surface resistance values between three common build orientations, as well as findings that establish a connection between increasing downskin laser power and a reduction in surface resistance for overhanging surfaces. Finally, additive manufactured rectangular waveguide sections are measured to assess the influence of combined upward and downward facing surfaces on surface resistance.

Liftoff Dielectric Resonator for the Microwave Surface Resistance Measurement of Metal Plates

A new method for accurate measurement of the microwave surface resistance of flat metal plates is proposed and verified experimentally, based on a sapphire dielectric resonator. System losses are accounted for by having continuous and controlled variation of the distance between the dielectric and the sample under test.

Measurement Technique for Microwave Surface Resistance of Additive Manufactured Metals

Additive manufactured (AM) metals are a subject of much interest for their performance in passive microwave applications. However, limitations could arise due to artifacts, such as surface texture and/or roughness resulting from the manufacturing process. We have, therefore, adopted a parallel plate microwave resonator for the accurate measurement of the surface resistance of flat metal plates, allowing for microwave current flow in two orthogonal directions by simply exciting a different resonant mode (at 5.3 and 6.4 GHz), without the need to remove and refix the sample. The systematic and random errors associated with the measurement of surface resistance are very small, less than 1% and 0.1%, respectively. The technique is demonstrated with measurements on a range of samples of the alloys, AlSi10Mg and Ti6Al4V, manufactured by laser powder bed fusion, in addition to traditionally machined samples of bulk metal alloys of aluminum and brass. For AM samples of AlSi10Mg, we have studied the effect on the surface resistance of directional roughness features, generated by the laser raster paths, in directions transverse or parallel to microwave current flow. Importantly for passive microwave device applications, we demonstrate that these samples exhibit no systematic anisotropy of surface resistance associated with such surface features.

Homogeneity in the Microwave Surface Resistance of Large YBa2Cu3O7−δ Superconductor Films Coated with Au

We studied homogeneity in the microwave surface resistance of Au-coated YBa2Cu3O7−δ (Au/YBCO) films. The Au/YBCO film with 4-inches in diameter was prepared in-situ with the thicknesses of 68 nm and 365 nm for the Au layer and the YBa2Cu3O7−δ (YBCO) layer, respectively. We measured the effective surface resistance (RS,eff) of the Au/YBCO film at temperatures of 25 K, 60 K, 70 K, and 77 K, respectively, at 29 different positions on the film. The RS,eff values were analyzed with the finite thicknesses of the Au and the YBCO layers taken into consideration. The local RS,eff values of the Au/YBCO film appeared to be almost uniform at 25 K. Meanwhile significant positional dependences were observed for the RS,eff of the Au/YBCO film at 60 K, 70 K and 77 K, respectively, which seemed to be due to non-uniform local penetration depth (λ) of the YBCO layer rather than due to variations in the local real-part conductivity of the YBCO layer. The observed results could not be attributed to the properties of the Au layer. At temperatures below the critical temperature of YBCO, the observed RS,eff values of the Au/YBCO film were significantly lower than those estimated using the surface resistance of Au and YBCO, which reflected proximity effects of the normal-metal Au layer in contact with the superconducting YBCO layer. Homogeneity in the RS,eff of the Au/YBCO film might be correlated with the distributions in the local λ of the YBCO layer, providing valuable information needed for designing microwave filters.

High-efficiency deposition of 2-in. double-sided YBCO thin films in batch with pulsed inject MOCVD

A special substrate turnplate and a U-type nozzle were designed for efficient producing double-sided YBCO thin films in batch with pulsed inject MOCVD. In this paper, 12 pieces of 500 nm thick YBCO thin films could simultaneously deposited on both sides of 2-in. LaAlO3 single crystalline wafers in less than 4 h, at an average preparation rate of 16 min per piece. Deposited with the same condition, the double-sided YBCO thin films in one turn and different turns have good consistency. And the films maintained good homogeneity in-plane and consistency for both sides. Meanwhile, the critical current density is mainly distributed between 2.2 and 2.4 MA•cm−2 (77 K, 0 T), and the microwave surface resistance is only 0.323 mΩ (77 K, 10 GHz). The film properties could meet the commercial demand of microwave filters.

Microwave Response of Nanostructured High-Tc Superconductor Thin Films

A model for the microwave response of a nanostructured high-Tc superconductor (HTS) film, with implanted nanoparticles and nanorods of a dielectric material or point-like and columnar irradiation defects with a nano-sized cross-section is developed. In this case, the microwave surface resistance Rs(T,H,ω) is calculated both for the Meissner and mixed states of a superconductor film in an applied dc magnetic field. The obtained results indicate that the implantation of dielectric nanoparticles or point-like radiation defects can significantly improve superconductor characteristics at microwave frequencies. Namely, these nano-sized structural defects can decrease the surface resistance in the Meissner state and eliminate the oscillations of Abrikosov vortices and the related microwave energy losses, thus decreasing the contribution of Abrikosov vortices to the Rs value in the mixed state of a HTS film.

Residual stress and strain on microstructure evolutions and superconductivity of REBa2Cu3O7−δ films grown by trifluoroacetate metal organic deposition

This research studies the effects of the residual stress and strain on the microstructure, critical current density, transition temperature, and microwave surface resistance of REBa2Cu3O7−δ (RE, rare earth) thin films grown by trifluoroacetate metal organic deposition. Different concentrations of Dy are introduced into YBa2Cu3O7−δ superconducting thin films. The superconducting transition temperature increases with an increase in the internal strain, while the critical current density exhibits an inverse variation relationship with the residual stress and c-axis length. The current carrying capability of REBa2Cu3O7−δ thin films can be improved by doping with appropriate Dy concentrations, which can be determined based on the residual stress modulation in the REBa2Cu3O7−δ thin films. In addition, no apparent transition of tensile or compressive stresses is observed in the thin films during the residual stress measurements. This paper provides a useful theoretical guidance for effectively improving the superconductivity of REBa2Cu3O7−δ thin films.

Effects of Ageing on the Microwave Surface Resistance of MgB2 Superconductor Films Stored in Low Vacuum

Nb is a very popular material in superconductive electronic devices such as superconducting single-photon detectors and radio-frequency (RF) cavities for particle accelerators. Before MgB2 can be used as a substitute material, the stability of its properties over time must be confirmed. We studied how ageing over a period of 514 weeks affected the microwave surface resistance (RS) and the normal-state resistivity (ρ) of epitaxially-grown MgB2 films. MgB2 films with thicknesses of 1.0 μm and 1.7 μm were prepared using the hybrid physical CVD method, and exhibited a critical temperature (TC) of 39 K. They were stored in low-vacuum to prevent reactions with humidity, except when the films were taken out of storage for measurement purposes. Variation in RS with the ageing period appeared to be significant at very low temperatures. The RS values of 1.0 μm-thick and 1.7 μm-thick MgB2 films increased by ~ 8 times and ~ 3 times, respectively, over ~ 10 years. The aged MgB2 films appeared to have reduced π-band gap energy values. The pristine MgB2 films value of 2.3 meV was reduced to 1.3–1.7 meV over ~ 10 years. The TC of the MgB2 films remained almost the same regardless of the ageing time. The enhanced RS and ρ in the aged MgB2 films were attributed to increased intraband scattering within the π-band and the σ-band.

Characterizing the Distribution of Microwave Surface Resistance of HTS Film Based on Metal Ring

This paper presents a novel method for measuring the microwave surface resistance distribution of high-temperature superconducting thin films by a dielectric resonator with a metal ring. After calibration, the effect of metal ring loss on measurement accuracy is negligible. The measurement resolution is determined by the internal diameter of the metal ring, which makes the design of the device more flexible. Therefore, measurement with high resolution and convenience can be achieved simultaneously by using a metal ring with proper size and operating at a relatively low frequency. A test device working at 32 GHz with TE01(1 + δ) mode was developed. Its resolution is a circular area with a diameter of 5 mm and an area of 19.6 mm2. The unloaded quality factor of the test device is more than 140,000, when it is loaded by the calibration cavity. The test device has a relative uncertainty of less than 6%, which complies with the uncertainty (20%) specified by the international standard for the microwave surface resistance measurement of high-temperature superconducting (HTS) thin film. The surface resistance distribution of a 2-in. YBCO/LAO/YBCO superconducting film sample was measured by this test device.

Correction to: The Effects of Gd-Free Impurity Phase on the Aging Behavior for the Microwave Surface Resistance of Ag-coated GdBa2Cu3O7−δ at Cryogenic Temperatures

The original version of this article unfortunately contained a mistake. Figure 9 was incorrect. The corrected figure is given below.

The Effects of Gd-Free Impurity Phase on the Aging Behavior for the Microwave Surface Resistance of Ag-coated GdBa2Cu3O7−δ at Cryogenic Temperatures

High-TC GdBa2Cu3O7−δ (GdBCO) superconductor has been popular for making superconductive tapes that have much potential for various fields of large-scale applications. We investigated aging effects on the microwave surface resistance (RS) of Ag-coated GdBCO layer on Hastelloy substrate, so called GdBCO coated conductors (CCs), and Ag-coated GdBCO films on LaAlO3 (LAO) single-crystal substrates at cryogenic temperatures and compared them with each other. Unlike the RS of Ag-coated GdBCO films showing significant degradation in 4 weeks, no significant aging effects were found in our Ag-coated GdBCO CCs aged 85 weeks. The reactive co-evaporation deposition and reaction (RCE-DR) method was used for preparing the Ag-coated GdBCO CCs. Such durability of the Ag-coated GdBCO CCs in terms of the RS could be explained by existence of a protective impurity phase, i.e., Gd-free Ba–Cu–O phase as confirmed by transmission electron microscopy study combined with the energy-dispersive X-ray spectroscopy measurements. Although the scope of this study is limited to the Ag-coated GdBCO CCs prepared by using the RCE-DR method, our results suggest that a solution for preventing the aging effects on transport properties of other kinds of Ag-coated GdBCO CCs could be realized by means of an artificially-grown protective impurity layer.

MOCVD-derived GdYBCO tapes with smooth surface and low Rs based on a new self-heating technology

The metal organic chemical vapor deposition (MOCVD) method was used to prepare GdYBCO films on LaMnO3/ homo epitaxial-MgO/ ion-beam-assisted-deposition-MgO/ solution-deposition-planarization-Y2O3 buffered Hastelloy tapes. By adopting a simple self-heating technique, the substrates were heated by the joule effect after applying a heating current (Ih) through Hastelloy metal tapes. The effects of substrate temperature and (Gd, Y)/Ba ratio (rc) in the precursor on the biaxial texture, surface morphology and superconducting performance of GdYBCO films were systematically investigated by varying the values of Ih and rc. Needle-like outgrowths formed on the substrate surface were characterized using a scanning electron microscope, energy dispersive spectrometer and X-ray diffraction system. The results show that a high Ih or rc leads to the formation of needle-like outgrowths. Therefore, Ih and rc are crucial process parameters that control the growth of needle-like outgrowths on the surface of GdYBCO films. Three hundred nanometer thick GdYBCO films were prepared at different Ih and rc by the MOCVD process. At an Ih of 27.0 A and an rc of 0.6, the surface of the GdYBCO film was very smooth and dense, which can provide a good template for multiple depositions of GdYBCO films. The critical current density of the deposited 300 nm-thick GdYBCO film was 4.4 MA/cm2 (77 K, 0 T), which is attributed to good biaxial texture and appropriate film composition. Furthermore, the microwave surface resistance (77 K, 10 GHz) of the GdYBCO film was merely 0.581 mΩ.