Maria Sparing

Investigating the Outskirts of Fe and Co Catalyst Particles in Alumina-Supported Catalytic CVD Carbon Nanotube Growth

Using thermal CVD, the synthesis of multi-walled carbon nanotubes exhibiting roots anchored directly onto alpha-alumina supports, rather than the catalyst particle, is reported. At such roots, the alignment of the graphitic planes with the support lattice fringes depends on the support crystal structure and orientation. Surface defects may alter the reactivity of the surface or control the anchoring of supported atoms or nanoparticles. We argue this surface defect is provided by the catalyst particles edge interaction with the support, in other words its circumference. The development of oxide-based catalysts is attractive in that they potentially provide an appropriate solution to directly integrate the synthesis of carbon nanotubes and graphene into silicon-based technology.

Catalyst Poisoning by Amorphous Carbon during Carbon Nanotube Growth: Fact or Fiction?

The influence of amorphous carbon on FePt catalyst particles under chemical vapor deposition conditions typically applied for CNT growth is examined through two routes. In the first, FePt catalyst particles supported on alumina are exposed to a well-established cyclohexane thermal CVD reaction at various temperatures. At higher temperatures where self-pyrolysis leads to copious amorphous carbon and carbon tar formation, carbon nanotubes are still able to form. In the second route, an amorphous carbon film is first deposited over the catalyst particles prior to the CVD reaction. Even for reactions where further amorphous carbon is deposited due to self-pyrolysis, graphitization is still demonstrated. Our findings reveal that the presence of amorphous carbon does not prevent catalytic hydrocarbon decomposition and graphitization processes. We also show an additional catalytic reaction to be present, catalytic hydrogenation, a process in which carbon in contact with the catalyst surface reacts with H(2) to form CH(4).

Superconducting Magnetic Bearing as Twist Element in Textile Machines

Superconducting magnetic bearings (SMBs) enable the levitation of a magnet in a stable position over a cooled superconductor without the need for an additional positioning system. These passive bearings are being investigated for applications wherever a stationary levitation or a contact-free rotary or linear motion is desired and cannot be realized by simpler means. Ring spinning is the most widely used technique for the production of short staple yarn in the textile industry. The productivity of the ring-spinning process depends on the rotational speed of the spindle. It is limited by friction in the so-called ring-traveler twist element. During the spinning process, the traveler is dragged along the ring by the yarn with up to 30000 r/min. The resulting friction causes wear of the twist element and melting of synthetic yarn at high spindle speeds due to strong heat generation. Recently, an SMB was implemented as a contact-free twist element in a ring-spinning machine up to 11000 r/min. This prototype SMB consists of a rotating permanent-magnetic (PM) NdFeB ring acting as a yarn-driven traveler and a stationary bulk YBCO ring at 77 K in an open LN2 bath. In this paper, we investigate the bearing properties of this SMB with regard to the special requirements of the ring-spinning process. Dynamics of the spatially vibrating and rotating PM ring are analyzed including the forced vibration due to interaction with the yarn. For the expected yarn forces, the displacement amplitude at the resonance frequency remains below the field-cooling height. In addition, the possibility of realizing higher rotational speeds by mechanically reinforcing the NdFeB ring with a shrunk-on steel shell is discussed.

Innovative twisting mechanism based on superconducting technology in a ring-spinning system:

Twist plays an important role to impart tensile strength in yarn during the spinning process. In the most widely used ring-spinning machine for short staple yarn production, a combination of ring and traveler is used for inserting twist and winding the yarn on cops. The main limitation of this twisting mechanism is the friction between the ring and traveler, which generates heat at higher speed and limits the productivity. This limitation can be overcome by the implementation of a magnetic bearing system based on superconducting technology, which replaces completely the existing ring/traveler system of the ring-spinning machine. This superconducting magnet bearing consists of a circular superconductor and permanent magnet ring. After cooling the superconductor below its transition temperature, the permanent magnet ring levitates and is free to rotate above the superconductor ring according to the principles of superconducting levitation and pinning. Thus the superconducting magnetic bearing (SMB) ensures a friction-free operation during spinning and allows one to increase spindle speed and productivity drastically. The yarn properties using the SMB system have also been investigated and they remain nearly identical to those of conventional ring yarns.

Dynamics of rotating Superconducting Magnetic Bearings in Ring Spinning

A superconducting magnetic bearing (SMB) consisting of a stationary superconductor in a ring-shaped flowthrough cryostat and a rotating permanent-magnetic (PM) ring is investigated as a potential twist element in the textile technological process of ring spinning. Since the dynamic behavior of the rotating PM influences the yarn and the stability of the spinning process, these factors are studied in this paper considering the acting forces of the yarn on the PM-ring, its vibration modes, and the resulting oscillation amplitudes. For the assessment of a safe field-cooling distance during the operation of the rotating SMB in a ring-spinning machine, a correct calculation of the resonance magnification is particularly important. Therefore, the decay constant δ of the damped oscillation was measured as a function of the field-cooling distance (FCD) and the displacement. The observed increase of the decay constant δ with the initial lateral displacement and decreasing FCD is discussed in correlation to the number of depinned flux lines.

Ba 2 Y(Nb/Ta)O 6 –Doped YBCO Films on Biaxially Textured Ni–5at.% W Substrates

The incorporation of nanoscaled pinning centers in superconducting YBa₂Cu₃O_7-δ (YBCO) films is one of the core topics to enhance the critical current density J_c(B, Θ) of coated conductors. The mixed double-perovskite Ba₂Y(Nb/Ta)O₆ (BYNTO) can be grown in nanosized columns parallel to the YBCO c-axis and in steplike patterns, making it customizable to meet specific working conditions (T, B, Θ). We compare a 1.6 μm thick film of pure YBCO and a similar film with additional 5 mol% of BYNTO, grown by pulsed laser deposition with a growth rate of 1.6 nm/s on chemically buffered biaxially textured Ni-5at.% W tape. Our doped sample shows nanosized BYNTO columns parallel to c_YBCO and plates in the ab-plane containing Y, Nb, and Ta. An improved homogeneity of the critical current density J_c over the sample was evaluated from trapped field profiles measured with a scanning Hall probe microscope. The mean J_c in the rolling direction of the tape is 1.8 MA/cm² (77 K, self-field) and doubles the value of the undoped sample. Angular-dependent measurements of the critical current density, J_c(Θ), show a decreased anisotropy of the doped film for various magnetic fields at 77 K and 64 K.

Cryogenic System for the Integration of a Ring-Shaped SMB in a Ring-Spinning Tester

A liquid nitrogen cryostat has been developed for experiments with a Superconducting Magnetic Bearing (SMB) in a ring spinning machine. The use of an SMB without friction instead of the common friction afflicted twist element is a promising approach leading to increased productivity and material variety. Here, an SMB designed as an axial ring system consisting of a liquid nitrogen (LN2)-cooled high-temperature superconductor ring YBa2Cu3O7-x and a permanent-magnet ring at room temperature is investigated. To assure the high quality of the yarn, the process has strong edge conditions concerning the air moisture and the ambient temperature. To protect the yarn from LN2 temperature, a continuous-flow cryostat was designed, tested, and integrated in the ring spinning tester. The geometric requirements of the spinning process lead to a ring-shaped vessel with a hole in the middle for the rotating spindle. First spinning experiments running at rotations in the range of 20 000 rpm show a reliable operation of the continuous flow cryostat in the SMB-based spinning process. In addition, easy process handling and low LN2 consumption are demonstrated.

-Doped Thick Films on Highly Alloyed Textured Ni-W Tapes

YBa₂Cu₃O_7-δ (YBCO) films with a thickness of up to 2 μm containing nano-sized BaHfO₃ (BHO) have been grown on biaxially textured metal tapes by pulsed laser deposition. Transmission electron microscopy revealed the inclusion of Y₂O₃ platelets and rod-like BHO structures. A c-axis oriented growth of the YBCO layer and biaxial oriented incorporation of BHO is confirmed by X-Ray diffraction. BHO expands the c-axis of YBCO, flattens the surface of YBCO and densifies the films. Y₂O₃ inclusions get smaller and finer distributed, possibly with a positive influence on pinning effects. A superconducting transition temperature T_c of about 89 K was determined, decreasing slightly with increasing BHO content. Transport current measurements in the maximum Lorentz force configuration showed an increased irreversibility field H_irr and higher pinning force density F_p with increasing BHO content in high magnetic fields.

Mathematical Modeling of Dynamic Yarn Path Considering the Balloon Control Ring and Yarn Elasticity in the Ring Spinning Process Based on the Superconducting Bearing Twisting Element

The productivity of the conventional ring spinning process is currently limited by the frictional heat that occurs in the ring/traveler twisting system. In the framework of a fundamental research project from the German Research Foundation (DFG), the levitation principle of superconducting magnetic bearing (SMB) was implemented as a twisting element in order to eliminate the frictional problem and thus aim, at least, to double the productivity. A mathematical model of the dynamic yarn path has already been presented considering the friction free SMB system up to an angular spindle speed of 25.000 r.p.m. In this paper, the existing theoretical model, which was developed up to 25.000 r.p.m, was further modified considering the balloon control ring and yarn elasticity at a higher angular spindle speed, such as 50.000 r.p.m. The model was solved numerically using the Runge-Kutta method. With this model, it is possible to estimate the yarn tension distribution and balloon form considering the above-mentioned parameters. The model established was further validated by comparing the yarn tension and balloon forms predicted with measured ones up to an angular spindle speed of 15.000 r.p.m in a ring spinning tester based on superconducting magnetic bearing.

Mathematical modeling, simulation and validation of the dynamic yarn path in a superconducting magnet bearing (SMB) ring spinning system

The new concept of a superconducting magnetic bearing (SMB) system can be implemented as a twisting element instead of the existing one in a ring spinning machine, thus overcoming one of its main frictional limitations. In the SMB, a permanent magnet (PM) ring rotates freely above the superconducting ring due to the levitation forces. The revolution of the PM ring imparts twists similarly to the traveler in the existing twisting system. In this paper, the forces acting on the dynamic yarn path resulting from this new technology are investigated and described with a mathematical model. The equation of yarn movement between the delivery rollers and the PM ring is integrated with the Runge-Kutta method using MATLAB. Thus, the developed model can estimate the yarn tension and balloon form according to different spindle speeds considering the dynamic behavior of the permanent magnet of the SMB system. To validate the model, the important relevant process parameters, such as the yarn tension, are measured at different regions of the yarn path, and the balloon forms are recorded during spinning with the SMB system using a high speed camera.