Herman K. Hemmes

Noise properties of direct current SQUIDs with quasiplanar YBa2Cu3O7 Josephson junctions

We describe the noise performance of dc SQUIDs fabricated with quasiplanar ramp‐type Josephson junctions on the basis of c‐axis‐oriented YBa2Cu3O7/PrBa2Cu3O7 thin‐film heterostructures. The noise spectrum of the dc SQUIDs was measured with dc‐ and ac‐bias schemes at different temperatures and showed values below 10−5 Φ0/Hz1/2 down to frequencies of about 1 Hz at 70 K. Up to now for the magnetic fluxnoise and the energy resolution obtained at 1 kHz and 77 K the best values were 2.5×10−6, Φ0/Hz1/2 and 3×10−31 J/Hz, respectively. A study of the white and 1/fnoises of the SQUIDs was performed. The influence of magnetic flux, bias current, high static magnetic fields, and aging on the SQUID noise were investigated. The junctions and devices do not degrade due to aging in air or thermal cycling.

Interaction between plasma sprayed YBaCuO and nimonic substrates

The interaction of YBaCuO layers, deposited by atmospheric plasma spraying, with nimonic substrates at high temperature has been studied using X-ray photoelectron spectroscopy and Auger electron spectroscopy (AES). Both the region at the YBaCuO/ nimonic interface and the surface of clean nimonic substrates after annealing in vacuum and oxygen have been studied in terms of chemical composition and peak shapes. Chromium was detected more than 20 ?m deep in the YBaCuO layer. This is explained in terms of a chemical reaction of Cr3+ oxides from the nimonic with the YBaCuO. Depth profiling of the interface region combined with AES measurements reveals also an extensive migration of nickel oxides from the substrate into the YBaCuO.

Superconducting and structural properties of plasma sprayed YBaCuO layers deposited on metallic substrates

The properties of plasma sprayed Y-Ba-Cu-O coatings deposited on metallic substrates are studied. Stainless steel, nickel steels and pure nickel are used as substrate. Y-Ba-Cu-O deposited on stainless steel and nickel steel reacts with the substrate. This interaction can be suppressed by using an yttria-stabilized zirconia (YsZ) diffusion barrier. However, after heat treatment the Y-Ba-Cu-O layers on YsZ show cracks perpendicular to the surface. As a result the critical current density is very low. The best results are obtained for Y-Ba-Cu-O deposited on pure nickel; here no cracks perpendicular to the surface are observed. The critical current increases with the anneal temperature but annealing for longer than 10 h does not seem to improve the superconducting properties any further.

Plasma Sprayed High-Tc Layers for Magnetic Shielding

Plasma spraying is a well-known technique for the deposition of thick layers of ceramics and metals. Because of the large deposition rate large areas can be coated. Since the discovery of High-T, superconductors1,2 plasma spraying has also been applied to the fabrication of coatings of these materials.3–10 Large area coatings can be useful in magnetic shielding applications. The technical implementation of the plasma spray process and the subsequent thermal treatment for the production of High-Tc layers is relatively straightforward. However, the deposition process, the phase relations of the superconductors and the interactions with the substrate material make the whole process very complicated.

Time dependent critical state in disks and rings

The authors have developed a model to calculate the response of the current distribution in disks and rings to a time-dependent applied magnetic field. In the model, the ring (or disk) is divided into concentric segments. The segments are assumed to be inductively coupled to each other and to the applied field. A time-dependent magnetic field induces a finite electric field in the ring/disk. The induced currents will then depend on the magnitude of the electric field and the current-voltage (I-V) characteristic. The current-voltage characteristic is modeled by a nonlinear resistivity. The currents determined from AC magnetization measurements on rings and experimental I-V characteristics are compared with the results of the proposed model. It is found that the shape of the magnetization curves can be easily reproduced. However, the frequency dependence of the current in the ring shows a discrepancy. The experimental frequency dependence of the currents is much smaller than that expected on the basis of the I-V characteristics and the model calculations. A possible cause could be inhomogeneities in the sample influencing the current distribution.

Interaction between intra-grain currents and transport current in bulk YBaCuO☆

The hysteric magnetization curves of transport and grain currents are measured for a ring shaped sample by applying an a.c. magnetic field and using pickup coils. The dependence of the magnetization curves on the amplitude of the applied a.c. magnetic field can be explained by a simple model. This model takes into account the time dependence of the transport and the intra-grain current. We will show that the mutual interaction of the grain and transport currents cannot be neglected in our samples. At first the influence of an additional d.c. background field on the transport current seems quite unexpected, but this d.c. field dependence can be interpreted too within our model.

The influence of additives on the current carrying capacity of bulk YBa2Cu3OX

We studied the influence of using different powders and several additives on the shaping process and critical current density of sintered bulk YBa2Cu3OX. It was found that all additives used facilitated the shaping process. Silver and most organic additives influenced the critical current density only little, Zinc reduced it to zero, but an ethylenebisstearoylamide compound doubled the critical current density as compared to samples without additives.