Gregory Kozlowski

Sensitivity and resolution of evanescent microwave microscope

A near-field evanescent microwave microscope is based on a /spl lambda//4 coaxial TEM transmission-line resonator with a silver-plated tungsten tip extending through an end-wall aperture. This microwave microscope is used to characterize local electromagnetic properties of dielectrics, conductors, and superconductors. The resolution of the probe is verified experimentally by scanning etched gold lines on a sapphire substrate. A first-order sensitivity estimation generated from a unified equivalent circuit model of the probe and sample is investigated. The sensitivity inherent to the resonant probe and system noise is discussed. Experimental validation of sensitivity is given.

Quasipermanent magnets of high temperature superconductor - Temperature dependence

We report on persistent field in quasipermanent magnets of high temperature superconductors. Magnets composed of irradiated Y1+Ba2Cu3O7 trapped field Bt=1.52 T at 77 K and 1.9 T at lower temperature. However, the activation magnet limited Bt at lower temperature. We present data on Jc(H,T) for unirradiated materials, and calculate Bt at various T. Based upon data at 65 K we calculate Bt in unirradiated single grains at 20 K, and find that 5.2 T will be trapped for grain diameter d∼1.2 cm, and 7.9 T for d=2.3 cm. Irradiated grains will trap four times these values.

Structural investigations and magnetic properties of sol-gel Ni0.5Zn0.5Fe2O4 thin films for microwave heating

Nanocrystalline Ni0.5Zn0.5Fe2O4 thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology, magnetic, and microwave absorption properties of the films calcined in the 673–1073 K range were studied with x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, vibrating sample magnetometry, and evanescent microwave microscopy. All films were uniform without microcracks. Increasing the calcination temperature from 873 to 1073 K and time from 1 to 3 h resulted in an increase of the grain size from 12 to 27 nm. The saturation and remnant magnetization increased with increasing the grain size, while the coercivity demonstrated a maximum near a critical grain size of 21 nm due to the transition from monodomain to multidomain behavior. The complex permittivity of the Ni–Zn ferrite films was measured in the frequency range of 2–15 GHz. The heating behavior was studied in a multimode microwave cavity at 2.4 GHz. The highest microwave heating rate in the temperature range of 315–355 K was observed in the film close to the critical grain size.

Development of nickel alloy substrates for Y-Ba-Cu-O coated conductor applications

Fabrication of long-length, textured substrates constitute a critical step in the successful application of coated High Temperature Superconductors (HTS). Substrate materials stronger than nickel are needed for robust applications, while substrates with non-magnetic characteristics are preferred for AC applications. The present work is thus focused on development of texture in high strength, non-magnetic substrate materials. As the development of cube texture is easier in medium to high stacking fault energy materials, binary alloys based on nickel were evaluated for the present application. High purity alloys were melted and hot/cold worked to obtain thin tapes. The development of texture in these alloys as a function of processing parameters was studied by X-ray diffraction and metallographic techniques. Orientation Imaging Microscopy (OIM) was used to quantify the extent of texture development in these substrates. Results to date on the development of texture by thermo-mechanical processing of these alloys are presented.

Principles of application of high temperature superconductors to electromagnetic launch technology

A review is presented of advances in the performance of bulk high-temperature superconductors (HTSC) which permit conductor and magnet development at practical magnetic fields to be pursued for high-current applications such as electromagnetic launchers (EMLs). While early hopes for a superconductor critical temperature (T/sub c/) approaching room temperature have not been fulfilled, numerous HTSC with T/sub c/ between 60 K and 125 K exist which can be successfully processed. Some of these HTSC are well enough understood that small conductors and coils may be fabricated for operation near 20 K. Numerous physics, magnetic flux mechanics, materials processing, and structural support issues remain for resolution before large-scale coils made of HTSC can be operated at high energy storage density at temperature well above 20 K. Properties and materials processing of HTSC and their relation to EML applications technology are described. >

Local complex permittivity measurements of porcine skin tissue in the frequency range from 1 GHz to 15 GHz by evanescent microscopy

The near-field evanescent microwave microscope is based on a coaxial transmission line resonator with a silver plated tungsten tip protruding through an end-wall aperture. The sensor is used to measure the local dielectric properties of porcine skin in the frequency range from 1 GHz to 15 GHz. The dielectric property of the skin within the near field of the tip frustrates the electric field and measurably changes the transmission lines resonant frequency and quality factor (Q). The shift of the resonators frequency and Q is measured as a function of tip-sample separation, and a quantitative relationship between the real and imaginary parts of the local dielectric constant using the method of images is established. The associated changes in quality factor image scans of subsurface tissue structure and dielectric properties of skin surface lesions are presented.

Local Dielectric and Strain Measurements in YBa2Cu3O7−δ Thin Films by Evanescent Microscopy and Raman Spectroscopy

The near-field evanescent microwave microscope is based on a coaxial transmission line resonator with a silver-plated tungsten tip protruding through an end-wall aperture. The sensor is used to measure local dielectric properties of thin-film YBa2Cu3O7?? deposited on three different SrTiO3 bi-crystal substrates having mismatch grain boundary angles of 3?, 6?, and 12?. The measurements in the superconducting state are below critical temperature at T = 79.4?K. The dielectric property of the superconductor within the near field of the tip frustrates the electric field and measurably changes the transmission lines resonant frequency. The shift of the resonators frequency is measured as a function of tip?sample separation and associated changes in quality factor (?Q) image scans of the thin film are presented. A quantitative relationship between the real and imaginary parts of the local dielectric constant and the frequency shift using the method of images is established. The comparison between experimental data and theory based on this method is given and discussed. Raman measurements of the intergranular strain within the YBa2Cu3O7?? thin film deposited on each SrTiO3 substrate in the region of the bi-crystal junction showed excellent correlation between grain boundary mismatch and induced grain boundary strain. Compressive strains normal to the a axis (i.e.?tensile strains normal to the b axis) were detected across the grain boundary. The magnitude of induced strain as well as its spread away from the grain boundary increased as the mismatch angle increased.

Influence of magnetic materials on the transport properties of superconducting composite conductors

Magnetic materials can help to improve the performance of practical superconductors on the macro/microscale as magnetic diverters and also on the nanoscale as effective pinning centres. It has been established by numerical modelling that magnetic shielding of the filaments reduces ac losses in self-field conditions due to decoupling of the filaments and, at the same time, it increases the critical current of the composite. This effect is especially beneficial for coated conductors, in which the anisotropic properties of the superconductor are amplified by the conductor architecture. However, ferromagnetic coatings are often chemically incompatible with YBa2Cu3O7 and (Pb,Bi)2Sr2Ca2Cu3O9 conductors, and buffer layers have to be used. In contrast, in MgB2 conductors an iron matrix may remain in direct contact with the superconducting core. The application of superconducting–magnetic heterostructures requires consideration of the thermal and electromagnetic stability of the superconducting materials used. On the one hand, magnetic components reduce the critical current gradient across the individual filaments but, on the other hand, they often reduce the thermal conductivity between the superconducting core and the cryogen, which may cause the destruction of the conductor in the event of thermal instability. A possible nanoscale method of improving the critical current density of superconducting conductors is the introduction of sub-micron magnetic pinning centres. However, the volumetric density and chemical compatibility of magnetic inclusions has to be controlled to avoid suppression of the superconducting properties.

Characterization of local dielectric properties of superconductor YBa2Cu3O7-δ using evanescent microwave microscopy

A near-field evanescent microwave microscope based on a coaxial transmission line resonator with a tungsten tip protruding through an end-wall aperture is used to measure local dielectric properties of thin film YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// in superconducting state below critical temperature T/sub c/=91 K at T=79.4 K and in normal state at room temperature (T=298 K). The dielectric property of the superconductor within the near field of the tip frustrates the electric field and measurably changes the transmission lines resonant frequency. The shift of the resonators frequency is measured as a function of tip-sample separation and associated change in quality factor (Q) image scans of the thin film is obtained. A quantitative relationship between the real and imaginary parts of the local dielectric constant and the frequency shift using the method of images is established. The comparison between experimental data and theory based on this method is given and discussed for YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film deposited on LaAlO/sub 3/ substrate.

YBa2Cu3O7−x–Ag thick films deposited by pulsed laser ablation

Abstract Ag-doped YBa 2 Cu 3 O 7− x films, with thickness ranging from 0.06 to 2.5 μ m, were deposited by pulsed laser ablation onto (100) LaAlO 3 single-crystal substrates. The target was YBa 2 Cu 3 O 7− x with 5 wt.% Ag addition. The presence of Ag in the films in concentrations of ∼1 at.% was detected by X-ray fluorescence and secondary ion mass spectrometry (SIMS) analysis. Biaxial alignment of the films was indicated by φ scans with full-width-half-maximum (FWHM) spread of 1–2° for various thicknesses. Utilizing a standard deposition process, most films showed a critical transition temperature ( T c ) >90 K as measured by the ac susceptibility technique. Film critical current densities ( J c ) on the order of 10 6 A/cm 2 were measured at 77 K with a four-probe technique on a 100- μ m-wide patterned microbridge.