Tunnelling in rare-earth nitride structures

<p>GdN thin film device structures, including magnetic tunnel junctions (MTJs), were grown by physical vapour deposition and their electrical properties were investigated. Growth compatibility between GdN and various contact metals (Al, Au, Gd and Nb) was assured using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. I developed a photomask and lithographic process to isolate electrical behaviour perpendicular to the plane of the films. Al and Au were confirmed to make ohmic contact to GdN, while Gd and Nb both formed Schottky-like barriers at the interface with GdN. In MTJ structures, device electrical characteristics were dominated by tunnelling behaviour through the GaN barrier layer. The Simmons model was successfully applied to tunnelling measurements of Al/GdN/GaN/GdN/Gd structured MTJs to determine the barrier properties. MTJs grown with Al bottom contacts were grown with 1.5eV potential barrier height and 2.5 nm width. Finally, MTJs contacted with Nb exhibited a large magnetoresistance (> 500%), greater than GdN-based MTJs recorded in the literature [Warring et al. ”Magnetic Tunnel Junctions Incorporating a Near-Zero-Moment Ferromagnetic Semiconductor”, Phys. Rev. Appl., vol.6, p.044002, 2016].</p>

Tunnelling in rare-earth nitride structures

<p>GdN thin film device structures, including magnetic tunnel junctions (MTJs), were grown by physical vapour deposition and their electrical properties were investigated. Growth compatibility between GdN and various contact metals (Al, Au, Gd and Nb) was assured using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. I developed a photomask and lithographic process to isolate electrical behaviour perpendicular to the plane of the films. Al and Au were confirmed to make ohmic contact to GdN, while Gd and Nb both formed Schottky-like barriers at the interface with GdN. In MTJ structures, device electrical characteristics were dominated by tunnelling behaviour through the GaN barrier layer. The Simmons model was successfully applied to tunnelling measurements of Al/GdN/GaN/GdN/Gd structured MTJs to determine the barrier properties. MTJs grown with Al bottom contacts were grown with 1.5eV potential barrier height and 2.5 nm width. Finally, MTJs contacted with Nb exhibited a large magnetoresistance (> 500%), greater than GdN-based MTJs recorded in the literature [Warring et al. ”Magnetic Tunnel Junctions Incorporating a Near-Zero-Moment Ferromagnetic Semiconductor”, Phys. Rev. Appl., vol.6, p.044002, 2016].</p>

Analyse der Wirkmechanismen im fluidfreien Wälzkontakt mit beschichteten Oberflächen

ZusammenfassungDer Zahnkontakt wird in Zahnradgetrieben in der Regel mit flüssigem Schmierstoff, wie beispielsweise Öl oder Fett geschmiert. Jedoch führen extreme Betriebsbedingungen, wie beispielsweise hohe Druck- und Temperaturschwankungen, bei flüssigen Schmierstoffen zum Ausfall der positiven Schmiereigenschaften. Für den Einsatz von Zahnradgetrieben unter fluidfreien Betriebsbedingungen bieten Festschmierstoffe eine alternative Schmierung im Wälzkontakt. Das Ziel dieser Arbeit ist die Kenntnis über die Wirkmechanismen im PVD-beschichteten (Physical Vapour Deposition – Physikalische Gasphasenabscheidung) und fluidfreien Wälzkontakt. Für den fluidfreien Einsatz im Zahnkontakt wurde ein Schichtsystem auf Basis von Molybdändisulfid im Zahnrad-Analogieversuch auf dem Zwei-Scheiben-Reibkrafttribometer untersucht. Anhand der Charakteristik der Reibkraft kann zwischen einer Gebrauchs- und Lebensdauer unterschieden werden. Dabei zeigt die Lebensdauer eine bessere Wiederholgenauigkeit als die Gebrauchsdauer. Für die Kenntnisse über die Verschleißmechanismen wurden in Bezug auf Gebrauchsdauer und Lebensdauer Versuche mit Intervallbetrieb durchgeführt und ausgewertet. Die Verschleißanalyse der Kontaktfläche auf der Prüfwelle zeigt, dass sich bereits zu Beginn einer instationären Reibungsphase im Prüflauf ein starker Verschleiß in einem großen Teil der Laufbahn entwickelt hat.

Modelling evaporation in electron-beam physical vapour deposition of thermal barrier coatings

This work presents computational models of ingot evaporation for electron-beam physical vapour deposition (EB-PVD) that can be applied to the deposition and development of thermal barrier coatings (TBCs). TBCs are insulating coatings that protect aero-engine components from high temperatures, which can be above the component’s melting point. The development of advanced TBCs is fuelled by the need to improve engine efficiency by increasing the engine operating temperature. Rare-earth zirconates (REZ) have been proposed as the next-generation TBCs due to their low coefficient of thermal conductivity and resistance to molten calcium-magnesium alumina-silicates (CMAS). However, the evaporation of REZ has proven to be challenging, with some coatings displaying compositional segregation across their thickness. The computational models form part of a larger analytical model that spans the whole EB-PVD process. The computational models focus on ingot evaporation, have been implemented in MATLAB and include data from 6 oxides: ZrO2, Y2O3, Gd2O3, Er2O3, La2O3 and Yb2O3. Two models (2D and 3D) successfully evaluate the evaporation rates of constituent oxides from multiple-REZ ingots, which can be used to highlight incompatibilities and preferential evaporation of some of these oxides. A third model (local composition activated, LCA) successfully predicts the evaporation rate of the whole ingot and replicates the cyclic change in composition of the evaporated plume, which is manifested as changes in compositional segregation across the coating’s thickness. The models have been validated with experimental data from Cranfield University’s EB-PVD coaters, published vapour pressure calculations and evaporation rate formulas described in the literature.

Effect of Coatings on the Performance of Injection Molds

It is known from theory that coatings on the forming surface increase the service life of injection molding molds. In practice, the most widespread method of nitriding, which has a number of undeniable advantages, while there are other promising coatings. Therefore, the article considered coatings applied by the physical vapour deposition method in comparison with nitriding. The comparison was carried out on the basis of pressure and temperature indicators on the walls of the forming surface of the working inserts of the molds.

The Principle of Choosing the Composition of the Protective Coating for Die Casting Molds

The article considers the requirements for protective coatings operating in the conditions of injection molding of non-ferrous metal alloys, among which the most important are the provision of crack resistance and wear resistance of the forming surface. It is revealed that single-layer coatings applied by the physical vapour deposition method, regardless of its composition, are not able to fully meet the formulated requirements. It is established that multilayer coatings provide increased performance of structural elements of molds in comparison with single-layer ones.

High hole mobility in physical vapour deposition-grown tellurium-based transistors

Carrier mobility is one of most important figures of merit for materials that can determine to a large extent the corresponding device performances. So far, extensive efforts have been devoted to the mobility improvement of two-dimensional (2D) materials regarded as promising candidates to complement the conventional semiconductors. Graphene has amazing mobility but suffers from zero bandgap. Subsequently, 2D transition-metal dichalcogenides benefit from their sizable bandgap while the mobility is limited. Recently, the 2D elemental materials such as the representative black phosphorus can combine the high mobility with moderate bandgap; however the air-stability is a challenge. Here, we report air-stable tellurium flakes and wires using the facile and scalable physical vapour deposition (PVD) method. The prototype field-effect transistors were fabricated to exhibit high hole mobility up to 1485 cm 2 V −1 s −1 at room temperature and 3500 cm 2 V −1 s −1 at low temperature (2 K). This work can attract numerous attentions on this new emerging 2D tellurium and open up a new way for exploring high-performance optoelectronics based on the PVD-grown p-type tellurium.