Shakil Awan

Explaining the self-field AC loss behaviour of silver-clad (Bi,Pb)-2223 tapes for power engineering applications

Abstract The AC losses of mono- and multi-filamentary (Bi,Pb)-2223 silver-sheathed tapes due to an applied AC current have been measured. The data are compared to the Norris equations for the AC loss of elliptical and thin-strip superconductors. Reasons for the observed differences between theory and measurement for these tapes and others in the literature are discussed. It is suggested that the main factors determining the self-field AC loss in such practical superconductors are the transverse J c distribution in monofilaments and the filament distribution in multifilamentary tapes. For the latter, evidence is provided that tapes with filaments arranged in columns have a lower loss. For the development of low loss conductors for power applications these factors need to be accurately controlled over long lengths, requiring much improvement to existing processing techniques.

Towards accurate measurement of the frequency dependence of capacitance and resistance standards up to 10 MHz

Progress toward an understanding of the frequency dependence of capacitance and resistance standards at frequencies up to 10 MHz is presented. A qualitative comparison is also made for capacitance and dissipation factor measurements between the National Physical Laboratory (NPL) high-frequency four terminal-pair (4TP) bridge and a commercial impedance analyzer for the first time. A set of novel high-frequency calculable coaxial resistance standards, of nominal 100 /spl Omega/ and 1 k/spl Omega/ values, have been developed and their calculated frequency dependence up to 1 MHz is given.

A new four terminal-pair bridge for traceable impedance measurements at frequencies up to 1 MHz

A new four terminal-pair bridge has been developed and used for traceable measurement of capacitance and resistance at frequencies up to 1 MHz. The apparent capacitance of a gas-filled 100-pF standard agrees with calculated values to better than 200 /spl mu/F/F at 1 MHz. This is an order of magnitude improvement on existing capabilities at NPL. The frequency dependence of the dissipation factor of capacitance standards is discussed, and the capacitance and dissipation factor of ceramic NPO-dielectric standards has also been measured. The frequency dependence measurements of the calculable coaxial 100-/spl Omega/ and 1-k/spl Omega/ resistance standards are also presented.

Optimized design and process for making a DC voltage reference based on MEMS

A micromechanical moving plate capacitor has been designed and fabricated for use as a dc voltage reference. The reference is based on the characteristic pull-in property of a capacitive microelectromechanical system (MEMS) component. The design is optimized for stability. A new silicon-on-insulator (SOI) process has been developed to manufacture the component. We also report on improved feedback electronics and the latest measurement results.

Transport Conductivity of Graphene at RF and Microwave Frequencies

We measure graphene coplanar waveguides from direct current (DC) to 13.5GHz and show that the apparent resistance (in the presence of parasitic impedances) has an quadratic frequency dependence, but the intrinsic conductivity (without the influence of parasitic impedances) is frequency-independent. Consequently, in our devices the real part of the complex alternating current conductivity is the same as the DC value and the imaginary part~0. The graphene channel is modelled as a parallel resistive-capacitive network with a frequency dependence identical to that of the Drude conductivity with momentum relaxation time~2.1ps, highlighting the influence of alternating current (AC) electron transport on the electromagnetic properties of graphene. This can lead to optimized design of high-speed analogue field-effect transistors, mixers, frequency doublers, low-noise amplifiers and radiation detectors.

Evaluation of coaxial bridge systems for accurate determination of the SI Farad from the DC quantum Hall effect

An evaluation of the four-terminal-pair equal power 100 : 1, quadrature and 10 : 1 coaxial bridge systems for the traceable determination of NPL 10 pF primary capacitance standard from the DC quantum Hall effect is reported. This was instigated to resolve the small but significant discrepancies observed in the NPL measurements during the recent EUROMET and CCEM comparisons. A formal bilateral comparison between NPL and BIPM was conducted (involving EUROMET two-terminal-pair and BIPM three-terminal capacitance standards) to establish a new degree of equivalence between the two laboratories for the 10 pF and 100 pF capacitance measurements at 1 kHz and 1.592 kHz. The majority of results show agreement within the estimated 2σ combined relative standard uncertainty of measurement of the two laboratories. However, the best agreement was obtained for the EUROMET two-terminal-pair 10 pF and 100 pF capacitance standards with a mean difference between NPL and BIPM results of (21.9 ± 117) nF/F and (51.4 ± 117) nF/F, respectively, at 1.592 kHz.

Self-field a.c. losses in mono- and multi-filamentary Bi-2223/Ag tapes at power frequencies

The measured self-field a.c. losses in mono- and multi-filamentary tapes with varying transport current, frequency and voltage tap configurations are compared to theoretical models. The observed losses are found to approach asymptotic values when the loop of the voltage taps is comparable to the width of the tapes. The measured quadrature electric field is found to be nonlinear with applied current. A model is presented to describe this nonlinearity for homogeneous elliptical and strip geometries. The possible significance of the model to the observed losses is also discussed.

Shielding technique for deposition of Au electrical contacts on graphene by sputtering

Here, the authors report on a novel shielding technique for the fabrication of electrical contacts on exfoliated graphene by sputtering and lift-off process. The technique solves this problem by removing unwanted gold film in patterning contacts and reduces the high contact resistivity typically found in sputtered devices ranging from 260 to 940 kΩ μm induced by sputtered Au on graphene. By using a shielding tube integrated into our sputtering machine and optimizing the sputtering parameters, contact resistivity as low as 1.04 kΩ μm has been achieved. Consequently, the total device resistivity is significantly reduced, and the yield rate of the devices fabrication has also increased from 17% to 90%.

Using electrical measurements to determine the self-field AC losses of cable models

Abstract The DC current distribution and self-field AC losses of three HTS cable models have been measured electrically. Each cable consisted of one or two layers of spirally wound Bi-2223 tape. Non-uniform distributions of the current were found, due to variations in terminal resistances and tape degradation during winding. This meant that caution was required in the placing of leads for measuring the in-phase voltage and in the resultant calculation of AC loss. Three different configurations for the voltage leads were employed and the disadvantages of each are discussed. The calculated AC losses are compared to theoretical models and interpreted, considering the influence of the non-uniform current distribution on the measurements.

Label-Free Sensors Based on Graphene Field-Effect Transistors for the Detection of Human Chorionic Gonadotropin Cancer Risk Biomarker

We report on the development of label-free chemical vapour deposition (CVD) graphene field effect transistor (GFET) immunosensors for the sensitive detection of Human Chorionic Gonadotropin (hCG), a glycoprotein risk biomarker of certain cancers. The GFET sensors were fabricated on Si/SiO2 substrate using photolithography with evaporated chromium and sputtered gold contacts. GFET channels were functionalised with a linker molecule to an immobile anti-hCG antibody on the surface of graphene. The binding reaction of the antibody with varying concentration levels of hCG antigen demonstrated the limit of detection of the GFET sensors to be below 1 pg/mL using four-probe electrical measurements. We also show that annealing can significantly improve the carrier transport properties of GFETs and shift the Dirac point (Fermi level) with reduced p-doping in back-gated measurements. The developed GFET biosensors are generic and could find applications in a broad range of medical diagnostics in addition to cancer, such as neurodegenerative (Alzheimer’s and Parkinson’s) and cardiovascular disorders.