arXiv subjects

S. P. Ringer

Publications and source records attributed to S. P. Ringer.

9 recordsLinked to original sources

arXiv2021

On the early stages of precipitation during direct ageing of Alloy 718

The Ni-based superalloy Alloy 718 is used in aircraft engines as high-pressure turbine discs and must endure challenging demands on high-temperature yield strength, creep-, and oxidation-resistance. Nanoscale $γ^{\prime}$- and $γ^{\prime \prime}$-precipitates commonly found in duplet and triplet co-precipitate morphologies provide high-temperature strength under these harsh operating conditions. Direct ageing of Alloy 718 is an attractive alternative manufacturing route known to increase the yield strength at 650 $^°$C by at least +10 $\%$, by both retaining high dislocation densities and changing the nanoscale co-precipitate morphology. However, the detailed nucleation and growth mechanisms of the duplet and triplet co-precipitate morphologies of $γ^{\prime}$ and $γ^{\prime \prime}$ during the direct ageing process remain unknown. We provide a correlative high-resolution microscopy approach using transmission electron microscopy, high-angle annular dark-field imaging, and atom probe microscopy to reveal the early stages of precipitation during direct ageing of Alloy 718. Quantitative stereological analyses of the $γ^{\prime}$- and $γ^{\prime \prime}$-precipitate dispersions as well as their chemical compositions have allowed us to propose a qualitative model of the microstructural evolution. It is shown that fine $γ^{\prime}$- and $γ^{\prime \prime}$-precipitates nucleate homogeneously and grow coherently. However, $γ^{\prime \prime}$-precipitates also nucleate heterogeneously on dislocations and experience accelerated growth due to Nb pipe diffusion. Moreover, the co-precipitation reactions are largely influenced by solute availability and the potential for enrichment of Nb and rejection of Al+Ti.

cond-mat.mtrl-sci↗

arXiv2019

In-plane antiferromagnetism in $Na_{0.5}CoO_2$ induced by $Sb_{Co}$ dopants

$Na_xCoO_2$ has a fascinating and complex magnetic phase diagram that can be further manipulated by doping. Here, we investigated the effect of electron doping on the magnetic coupling among $Co^{4+}$ ions in $Na_xCoO_2$ using density functional theory based on Hartree-Fock hybrid functional. We found that electron doping through substitutional $Sb_{Co}$ dopants flip the in-plane ferromagnetic coupling among $Co^{4+}$ ions in the undoped compound to antiferromagnetic. Electron doping through interstitial $Cu_{Int}$, however, does not have a similar effect as $Cu_{Int}$ dopant leaves the compound ferromagnetic, just like the case of the undoped compound. The results demonstrate the critical dependence of magnetic phase in $Na_{0.5}CoO_2$ on the dopant and its incorporation site.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic, electrochemical and thermoelectric properties of $P2 - Na_x(Co_{7/8}Sb_{1/8})O_2$

We theoretically investigated the electronic, electrochemical and magnetic properties of Sb doped $Na_xCoO_2$ ($x = 1, 0.75$ and $0.50$). $Sb_{Co}$ dopants adopt +5 oxidation state in $Na_xCoO_2$ host lattice for all Na concentrations ($x$). Due to high oxidation states, $Sb^{5+}$ strongly repels Na ions and therefore it decreases the electrochemical potential (vs. Na/Na$^+$). The electrons introduced by $Sb^{5+}$ localize on nearby Co ions creating $Co^{2+}$ species which are absent in undoped $Na_xCoO_2$. $Co^{2+}$ ions reduce the spin entropy flow decreasing the Seebeck coefficient in the Sb doped compounds. The results can be generalized to other dopants with high oxidation state.

cond-mat.mtrl-sci↗

arXiv2016

Intrinsic Ferromagnetism in the Diluted Magnetic Semiconductor Co:TiO$_2$

Here we present a study of magnetism in \CTO\ anatase films grown by pulsed laser deposition under a variety of oxygen partial pressures and deposition rates. Energy-dispersive spectrometry and transition electron microscopy analyses indicate that a high deposition rate leads to a homogeneous microstructure, while very low rate or postannealing results in cobalt clustering. Depth resolved low-energy muon spin rotation experiments show that films grown at a low oxygen partial pressure ($\approx 10^{-6}$ torr) with a uniform structure are fully magnetic, indicating intrinsic ferromagnetism. First principles calculations identify the beneficial role of low oxygen partial pressure in the realization of uniform carrier-mediated ferromagnetism. This work demonstrates that Co:TiO$_2$ is an intrinsic diluted magnetic semiconductor.

cond-mat.mtrl-sci↗

arXiv2014

Enhancement of Transition Temperature in FexSe0.5Te0.5 Film via Iron Vacancies

The effects of iron deficiency in FexSe0.5Te0.5 thin films (0.8<x<1) on superconductivity and electronic properties have been studied. A significant enhancement of the superconducting transition temperature (TC) up to 21K was observed in the most Fe deficient film (x=0.8). Based on the observed and simulated structural variation results, there is a high possibility that Fe vacancies can be formed in the FexSe0.5Te0.5 films. The enhancement of TC shows a strong relationship with the lattice strain effect induced by Fe vacancies. Importantly, the presence of Fe vacancies alters the charge carrier population by introducing electron charge carriers, with the Fe deficient film showing more metallic behavior than the defect-free film. Our study provides a means to enhance the superconductivity and tune the charge carriers via Fe vacancy, with no reliance on chemical doping.

cond-mat.supr-con↗

arXiv2014

On the roles of graphene oxide doping for enhanced supercurrent in MgB2 based superconductors

Due to their graphene-like properties after oxygen reduction, incorporation of graphene oxide (GO) sheets into correlated-electron materials offers a new pathway for tailoring their properties. Fabricating GO nanocomposites with polycrystalline MgB2 superconductors leads to an order of magnitude enhancement of the supercurrent at 5 K/8 T and 20 K/4 T. Herein, we introduce a novel experimental approach to overcome the formidable challenge of performing quantitative microscopy and microanalysis of such composites, so as to unveil how GO doping influences the structure and hence the material properties. Atom probe microscopy and electron microscopy were used to directly image the GO within the MgB2, and we combined these data with computational simulations to derive the property-enhancing mechanisms. Our results reveal synergetic effects of GO, namely, via localized atomic (carbon and oxygen) doping as well as texturing of the crystals, which provide both inter and intra granular flux pinning. This study opens up new insights into how low-dimensional nanostructures can be integrated into composites to modify the overall properties, using a methodology amenable to a wide range of applications.

cond-mat.supr-con↗

arXiv2011

Direct observation of local K variation and its correlation to electronic inhomogeneity in (Ba1-xKx)Fe2As2 Pnictide

Local fluctuations in the distribution of dopant atoms are a suspected cause of nanoscale electronic disorder or phase separation observed within the pnictide superconductors. Atom probe tomography results present the first direct observations of dopant nano-clustering in a K-doped 122-phase pnictides. First-principles calculations suggest the coexistence of static magnetism and superconductivity on a lattice parameter length scale over a large range of doping concentrations. Collectively, our results provide evidence for a mixed scenario of phase coexistence and phase separation originating from variation of dopant atom experiments distroibutions.

cond-mat.supr-con↗

arXiv2010

Graphene doping to enhance flux pinning and supercurrent carrying ability in magnesium diboride superconductor

It has been shown that graphene doping is sufficient to lead to an improvement in the critical current density - field performance (Jc(B)), with little change in the transition temperature in MgB2. At 3.7 at% graphene doping of MgB2 an optimal enhancement in Jc(B) was reached by a factor of 30 at 5 K and 10 T, compared to the un-doped sample. The results suggested that effective carbon substitutions by grapheme, 2D nature of grapheme and the strain effect induced by difference thermal coefficient between single grapheme sheet and MgB2 superconductor may play an important role in flux pinning enhancement.

cond-mat.supr-con↗

arXiv2008

Thermal strain-induced enhancement of electromagnetic properties in SiC-MgB2 composites

Strain engineering has been used to modify materials properties in ferroelectric, superconducting, and ferromagnetic thin films. The advantage of strain engineering is that it can achieve unexpected enhancement in certain properties, such as an increase in ferroelectric critical temperature, Tc, by 300 to 500K, with a minimum detrimental effect on the intrinsic properties of the material. The strain engineering has been largely applied to the materials in thin film form, where the strain is generated as a result of lattice mismatch between the substrate and component film or between layers in multilayer structures. Here, we report the observation of residual thermal stress/strain in dense SiC-MgB2 superconductor composites prepared by a diffusion method. We demonstrate that the thermal strain caused by the different thermal expansion coefficients between the MgB2 and SiC phases is responsible for the significant improvement in the critical current density, Jc, the irreversibility field, Hirr, and the upper critical field, Hc2, in the SiC-MgB2 composite where the carbon substitution level is low. In contrast to the common practice of improving the Jc and Hc2 of MgB2 through chemical substitution, by taking advantage of residual thermal strains we are able to design a composite, which shows only a small drop in Tc and little increase in resistivity, but a significant improvement over the Jc and Hc2 of MgB2. The present findings open up a new direction for manipulation of materials properties through strain engineering for materials in various forms.

cond-mat.supr-con↗