arXiv subjects

Gudrun Auffermann

Publications and source records attributed to Gudrun Auffermann.

6 recordsLinked to original sources

arXiv2021

Laser floating zone growth of SrVO$_3$ single crystals

The perovskite SrVO$_3$ is of interest for a variety of applications due to its simple metallic character and stability in reducing environments. Here we report the preparation of single-crystal SrVO$_3$ using the laser floating zone technique. Laue diffraction implies single domains ca. 30 mm in length. The stoichiometry of optimized crystals was found to be Sr$_{0.985}$VO$_{2.91}$ using inductively coupled plasma optical emission spectrometry and neutron powder diffraction analysis, with compositions adjustable depending on the crystal pulling rate. Heat capacity measurements from 2 to 300 K show variations with composition, attributable to a combination of impurity scattering and changes in phonon dynamics.Our results demonstrate the utility of the laser floating zone technique in preparing a range of materials, and our advances with SrVO$_3$ may help lead to applications including catalysis, transparent conducting oxides, thermionic emitters, and other electronic devices.

cond-mat.str-el↗

arXiv2020

Growth and transport properties of Mg3X2 (X = Sb, Bi) single crystals

The discovery of high thermoelectric performance in n-type polycrystalline Mg3(Sb,Bi)2 based Zintl compounds has ignited intensive research interest. However, some fundamental questions concerning the anisotropic transport properties and the origin of intrinsically low thermal conductivity are still elusive, requiring the investigation of single crystals. In this work, high-quality p-type Mg3Sb2 and Mg3Bi2 single crystals have been grown by using a self-flux method. The electrical resistivity \r{ho} of Mg3Bi2 single crystal displays an anisotropy with \r{ho} in-plane twice larger than out-of-plane. The low-temperature heat capacity and lattice thermal conductivity of Mg3Sb2 and Mg3Bi2 single crystals have been investigated by using the Debye-Callaway model, from which the existence of low-lying vibration mode could be concluded. Large Grüneisen parameters and strong anharmonicity are found responsible for the intrinsically low thermal conductivity. Moreover, grain boundary scattering does not contribute significantly to suppress the lattice thermal conductivity of polycrystalline Mg3Sb2. Our results provide insights into the intrinsic transport properties of Mg3X2 and could pave a way to realize enhanced thermoelectric performance in single-crystalline Mg3X2-based Zintl compounds.

cond-mat.mtrl-sci↗

arXiv2020

Large Nernst Power Factor over a Broad Temperature Range in Polycrystalline Weyl Semimetal NbP

The discovery of topological materials has provided new opportunities to exploit advanced materials for heat-to-electricity energy conversion as they share many common characteristics with thermoelectric materials. In this work, we report the magneto-thermoelectric properties and Nernst effect of the topological Weyl semimetal NbP. We find that polycrystalline, bulk NbP shows a significantly larger Nernst thermopower than its conventional thermopower under magnetic field. As a result, a maximum Nernst power factor of ~ 35*10-4 Wm-1K-2 is achieved at 9 T and 136 K, which is 4 times higher than its conventional power factor and is also comparable to that of state-of-the-art thermoelectrics. Moreover, the Nernst power factor maintains relatively large value over a broad temperature range. These results highlight that the enhancement of thermoelectric performance can be achieved in topological semimetals based on the Nernst effect and transverse transport.

cond-mat.mtrl-sci↗

arXiv2020

Synergistically creating sulfur vacancies in semimetal-supported amorphous MoS2 for efficient hydrogen evolution

The presence of elemental vacancies in materials is inevitable according to statistical thermodynamics, which will decide the chemical and physical properties of the investigated system. However, the controlled manipulation of vacancies for specific applications is a challenge. Here we report a facile method for creating large concentrations of S vacancies in the inert basal plane of MoS2 supported on semimetal CoMoP2. With a small applied potential, S atoms can be removed in the form of H2S due to the optimized free energy of formation. The existence of vacancies favors electron injection from the electrode to the active site by decreasing the contact resistance. As a consequence, the activity is increased by 221 % with the vacancy-rich MoS2 as electrocatalyst for hydrogen evolution reaction (HER). A small overpotential of 75 mV is needed to deliver a current density of 10 mA cm-2, which is considered among the best values achieved for MoS2. It is envisaged that this work may provide a new strategy for utilizing the semimetal phase for structuring MoS2 into a multi-functional material.

cond-mat.mtrl-sci↗

arXiv2020

In situ modification of delafossite type PdCoO2 bulk single crystal for reversible hydrogen sorption and fast hydrogen evolution

The observation of extraordinarily high conductivity in delafossite-type PdCoO2 is of great current interest, and there is some evidence that electrons behave like a fluid when flowing in bulk crystals of PdCoO2. Thus, this material is an ideal platform for the study of the electron transfer processes in heterogeneous reactions. Here, we report the use of bulk single crystal PdCoO2 as a promising electrocatalyst for hydrogen evolution reactions (HERs). An overpotential of only 31 mV results in a current density of 10 mA cm^(-2), accompanied by high long-term stability. We have precisely determined that the crystal surface structure is modified after electrochemical activation with the formation of strained Pd nanoclusters in the surface layer. These nanoclusters exhibit reversible hydrogen sorption and desorption, creating more active sites for hydrogen access. The bulk PdCoO2 single crystal with ultra-high conductivity, which acts as a natural substrate for the Pd nanoclusters, provides a high-speed channel for electron transfer

cond-mat.mtrl-sci↗

arXiv2019

Surface states in bulk single crystal of topological semimetal Co$_3$Sn$_2$S$_2$ towards water oxidation

The band inversion in topological phase matters bring exotic physical properties such as the emergence of a topologically protected surface states. They strongly influence the surface electronic structures of the investigated materials and could serve as a good platform to gain insight into the catalytic mechanism of surface reactions. Here we synthesized high-quality bulk single crystals of the topological semimetal Co$_3$Sn$_2$S$_2$. We found that at room temperature, Co$_3$Sn$_2$S$_2$ naturally hosts the band structure of a topological semimetal. This guarantees the existence of robust surface states from the Co atoms. Bulk single crystal of Co$_3$Sn$_2$S$_2$ exposes their Kagome lattice that constructed by Co atoms and have high electrical conductivity. They serves as catalytic centers for oxygen evolution process (OER), making bonding and electron transfer more efficient due to the partially filled $e_g$ orbital. The bulk single crystal exhibits outstanding OER catalytic performance, although the surface area is much smaller than that of Co-based nanostructured catalysts. Our findings emphasize the importance of tailoring topological non-trivial surface states for the rational design of high-activity electrocatalysts.

cond-mat.mtrl-sci↗